people on here know what hey got. people on CL will sell you KILLER bikes on the low low. this is what i do:
1) find out year, make, model, and engine.
2) make post about it on moped army.com.
3) go see the bike.
4) if it's running, ride it. allow it to warm up them see how fast it will go. listen to it, check brakes, tires, jankyness. if it souds like a can of scissors it ain't gonna be running long.
5) if it's dead (like most bikes i have come accross) them the haggling begins. try to turn the motor. it should spin and make a chuggy motor sound. if it's all locked up this can actually be good. you say "this motor appears to be seized." and they say something like "uh it ran fine when my grandmother rode it 15 years ago." this will allow you do be like "how about a little less loot." and sometimes (a lot) they are just stuck. make sure the bike is in some condition near fixable. look for rusty tank. look for bent wheels, missing parts, carburator? constantly whittling away at the price.
6) if the motor turns check for spark. read about this in your new best friend, the wiki. see if the lights work.
7) just be weird. when i go to get stuff i'll be all like "uhhhhhh.....hmmm.......weeeeeellll..........and this welllll,,,,hmmmm....gonna need a new one of these......ahhhh, and this looks broken." smell like whiskey. wear weird glasses. your that guy from the swamp that needs a moped.
8) if you like in a state that requires paper work make sure that's ok. if not at least get a bill of sale so if it comes up stolen your not liable.
9) buy it, or not.
10) make a post about it on MA.