There are cables that actuate the trunk release and the buttress releases. The plastic retention balls on the ends can fail and make it impossible to open the trunk normally.
Trunk won't open
Get a GM record pulled to verify the in service date and warranty remaining. Any GM dealer can do this for you.
You want to start with the local car because the Sky and Sol are highly susceptible to damage when lifted improperly. The front fenders crack frequently due to road vibration and are easily crushed if lifted with a floor jack. If there is damage you want to identify it before sale and with a dealer you can push to get them to repair the damage as part of the sale. There are basically no available right side factory fenders anymore and few left side fenders. They are repairable and there are aftermarket replacements available from DDM but you need to know if the fenders are cracked as this can be up to a $1k swinger on the retail price.
You also want to take a good look at the bottom front. Because of the configuration of the car nearly every new owner rams them into curbs one to several times and they get damaged. At the very least there will be scrapes on the bottom of the bumper cover that you can use to get a lower price or commitment to repair from a dealer that you probably will not get from a private party seller.
You also want to take a hard look at the top. More than a few cars have had problems with the folding mechanism that has damaged the canvas where it folds over the windows and / or damage to the canvas when it rubs against an improperly adjusted side window. The top is also susceptible to damage from the hold down feet mounted on the trunk lid. The rubber feet come off, fail or even twist allowing direct contact between the mounting structure and the top canvas while the top is in the trunk. I have seen more than one car that has holes in the top due to this. The canvas is $1600 plus installation which at a dealer can run into several hundred dollars. And to the best of my knowledge while there are plenty of replacement canvas parts available, there are no more top assemblies in stock.
You want to visually inspect the cup holders. The front one is notorious for failing and they cost upwards of $100 for replacement parts plus installation labor. The rear one between the seats is much more robust however they can and do get so full of dust and dirt that they will no longer deploy. If treated well they are fine but you do not know till you try to pull the things out.
Another area that I recommend you inspect is the center console. If the previous drivers did like me and leaned on them hard while driving and used them as a support while getting in and out of the car, they all will crack right down the center. I have had mine replaced under the warranty but you are probably out of warranty. The cover is easy to replace but again is in the range of $100 for a replacement part from GM. There are several threads that cover reinforcing below the cover so it will not crack again and DDM has a replacement that is much stonger and includes two cup holders but again that is in the same cost plus shipping and you get to install it.
If you are by chance looking at a 2008 RL, one factor is the production run for the first 6 or 8 months deleted the electronic vacuum pump that provided motive force for the brakes immediately after start up. The issue presents itself on some cars as high brake pedal force required and / or pulsing of the brake pedal for the first few seconds to the first couple of minutes upon cold start after sitting for an extended period or at high altitude. The issue is that the turbo car does not generate much vacuum (its boosted) and as a result once the vacume accumulator leaks down, there is no vacuum for the power brakes when you start. And when you start the factory tune, it is set to heat up the catalytic converter and not optimized for creating vacuum. There was a big safety investigation made and a lot of people had problems with this about mid-way through 2008 calendar year. My wife's car falls in this group and about once or twice a year we get the symptoms but it works fine through it and within about 30 seconds there is normal brake feel. The factory has a software update that if asked they will install on cars with this complaint and in extreme cases there is a retro fit kit that adds back on the electronic vacuum pump. But again, you cannot tell if the car you are looking at falls into this group or not unless you physically check for the pumps presence on the left side of the engine block, and unless you drive the car from a cold start. If you are buying from a dealer you could make the software update part of the deal and get a promise to do the pump retro fit under warranty because GM normally pays for it.
You want to cycle the driver’s seat up and down because a lot of cars have never had their electric adjusters cycled and they need to be freed up and lubricated before they work correctly. In fact, I did a test last year and out of 40 cars and 80 drivers only 5 knew that the driver’s seat went up and down with the switch! Servicing the seat is no big deal but it will be half an hour of labor and some parts allowance so its in the $100 range. If you buy from a dealer you are much more likely to have this addressed as part of the deal than if you buy from a private party. And if you do not have access to the car its difficult to verify operation.
Many, maybe most of the key fobs for 2006 through 2008 at least have a known manufacturing problem. The metal tab / connector that holds the battery in place has a cold solder joint. The fob starts to work intermittently and eventually stops working. If you are good with a solder iron you can easily fix it as many of us have done, but if not, then the dealer will charge you around $100 for a replacement fob and programming it with the car. Potentially times two. If you have access to the fobs, you can pop them open and inspect the condition of the at risk part and know if you are good to go or not. If not, again you are much more likely to get the fob replaced as part of the deal by a dealer.
The door sills have an aluminum insert that is glued to the plastic part. Unfortunately as a result of the attachment method and the difference in coefficient of thermal expansion between the aluminum trim insert and the plastic door sills, the aluminum part will almost always bow up around .25 inches in the middle during cold weather. In some instances drivers will drag their heels across the door sills and can catch the trim plate and bend the crap out of it. They are relatively cheap - around $25 plus labor to install, but if you find it after the fact then it’s your $25 and not the sellers.
If you are getting a GXP, put an OBD2 reader on it and pull any codes. There can be codes that are not displayed but that can indicate incipient problems.
Have the selling dealer pull the outlet air tube from the turbo at the turbo and check for oil. A little oil is normal. If there is a lot of oil there then have the breather valve checked. When they fail, and if the car is driven moderately hard they pump oil out of the valve cover into the intake at the turbo. If there is a LOT of oil there, have them check the drain on the intercooler to verify that it is not holding pooled oil there.
Check the intake clamps for condition. The factory clamps are basically radiator screw clamps and have been known to fail and allow leakage which can cause tuning issues under boost