the catch is that there's damage or wear that they're not telling you about, ..and don't have to, with the sold as is sign on it..
bring a mechanic along with you to check it out..there's always one good mechanic around somewhere, with their own shop, or working elsewhere and has a day off, hire them for about $100, which will be money well spent, and drive them to the dealer with you...let them check the vehicle over for you.
run the vin # through any online vin checker...it'll tell you the car history and any reports on accidents and repair, but that will also cost you about $20.
simply ask the dealer what known or unkonwn issues the vehicle may have..if they hedge on telling you, then find out for yourself, if you think the deal is worth it.
sometimes you luck up, sometimes you get a lemon...i bought a scooter from some classified that looked beat up..got it for $300..now all i see is that same model being sold everywhere for $1400, and all i needed was a light fixed...the beat up part was dirt and dust, i can't even tell people what i paid for it now, for fear of being mob lynched.