For one you do not know if its stolen, but if you suspect it is and its cheap buy it, get the guys ,rego,description and dont handle it to much (finger prints) etc ....then take it to police station and run the serial numbers etc if it is stolen the rightful owner gets it back/ person charged etc.
Now not always is some thing stolen because its cheap, pawn brokers offer $60- for $500 items every day.
Now lets try 128STEVES way, i get my stilh blower stolen and the thief asks Steve whether he would like to buy it cheap, Steves says no (he suspects its stolen) and dosn't want to be a party to this type of thing, the police tell him if he buys stolen property he is helping contribute to the problem.
So the thief sells it to the bloke around the corner, goes on he merry way happy got cash in his pocket and smile on his face, he does not get prosacuted and i will never see my blower again and he will steal again.
Steves got a smug smile on his face to as he tells everybody some one try to sell him suspected stolen property and he said no , you are a great bloke Steve but i will not be relying on you to get my stolen blower back ...unless you have a better idea ???.......and remember you cant go to the police because some one offered you some thing cheap.