If you unwittingly bought stolen goods, and the rightful owner can prove that it's their's then you have to return it. The police won't press charges, but there'll be a thorough investigation conducted, which translates into inconvenience for you since you'll have to explain how you got the item, from who etc. Also, you won't be compensated for your loss.
Here are some tips for those who're thinking of buying 2nd hand:
1)Always insist on seeing the IC and taking down the particulars of the person you bought the item from. That way, if there's ever a dispute over rightful ownership, you can refer the relevant authorities to the person you dealt with. Handphone no.s and addresses can change, but with an IC the person can always be tracked down.
2) Insist on getting at least a receipt for the item. Better yet original packaging. If there's no receipt cos the item's a redeemed gift, ask the seller to sign a purchase agreement. That will serve as proof that you got the item from a legitimate transaction, and not by other dubious means.
3) Do not assume that the item isn't stolen just because the guy can produce a receipt and original packaging. The item could have been stolen from someone's home. Ironically a lot of people keep their receipts in the boxes placed NEXT to or near their dry box. The burglar could've swiped the lot. Talk to the guy, see if he's familiar with the item he's selling, esp. he he claims to have used it.
At the end of the day, exercise caution. Most CSers here are legit, so there's a very low risk. Even then you don't want to be part of the odd statistic. It may seem a little awkward to ask for someone's IC, but if he's legit he should have no probs. And it'll save you loads of hassle if there are disputes later. And no you won't be sent to jail for unknowingly buying stolen stuff.