when you buy a stolen bike the only options you have is one: take the moral high road report the bike stolen and get the seller into trouble (in that case if there is no numbers on the bike its likley the true owner will never be contacted) and then he only gets done for theft of auto since its not woth over 10k
two: find out the bike is stolen.. sell the bike from a friends house from a web account that you will never use again (i recommend using a proxy) selling the bike off and recover a decent ammount of money so lets say 500 instead of 650 just so you re-coup and loose as little as possible..
third: part the bike out and takes much longer than option two but most people wont check the numbers on the engine and frame but anything with numbers i would recommend selling it in a fashion like number 2 but actually dont use a friends house use a random house and roll the bike there before the buyer rocks up.. if he seems to be worried that it does not look like your house sell it infront of a house where the garage door is up so it looks like you just come from there