I'd be cautious, even if they don't scam you the providence of their BTC might be dubious.
Why don't they sell on an exchange? Perhaps they acquired their Bitcoins via illegal means. Do you want the blockchain showing you as the new owner of those coins?
Same argument can be given about the FIAT in your pocket... Is it possible that $10 bill was once used in a sting operation to capture a drugdealer? Maybe it's marked? Maybe it has traces of cocaine?
https://themerkle.com/what-is-bitcoin-fungibility/True, but the only person who is going to be looking at that FIAT is the guy at the store you use to buy $10 worth of groceries from. Even if the authorities somehow figured out you withdrew that $10 from an ATM they aren't going to hold it against you.
Bitcoin, much like fiat, can be used for illegal activity.
A huge amount of cash has been used for shady activities, and the same applies to Bitcoin. To blame the person who receives the transaction would be absurd. In fact, if they were to do this then they wouldn't just blame you, but any merchants that you send BTC to as well.
The full force and strength of international law enforcement has the ability to look at the blockchain. Unfortunately they are likely to suspect (possibly unreasonably) that because you received the BTC from a scoundrel, that you must be a scoundrel yourself.
There's absolutely no reason for them to assume this. If they find the address of the criminal, they will attempt to track the coins in order to find any other illegal activities that the criminal was taking part in. It only matters if:
-The criminal was sending to sites such as Alpha Bay and using the coins for crime;
-The criminal was sending to their own wallets, in which case the authorities may attempt to seize the coins.
Even in the unlikely scenario that they bothered trying really hard and they tracked the coins to an individual, they would not arrest you - all you have to do is show them eBay where you bought the coins from and explain why you did so.