They're not co2 only but shoot a lot better on CO2 imho. If it seals up and shoots there's not too much else to check. Value mainly depends on the physical condition and to make sure all the parts are there. I would check to see if the site rail and safety are there. Those seem to be the commonly missing parts and you'll never find replacements. Also check the body where the barrel goes in. Make sure the tabs that hold the barrel in place look solid.
If there's a a pic or link, feel free to PM me and I can offer my opinion. Don't worry, I have no desire to try and backdoor it from ya.