Same as BitcoinAccepted here, im not going to be using anything ilegal. I don't want my coins to be mixed with coins that have illicit origin. Thats the problem of a public ledger, if you get some coins that are proved of illicit origin it may be a problem for you. Who would accept them as payment?
Well since BTC is not refundable it shouldn't be a problem, but it still hurts its fungible features.
There's no way to tell where your coins have come from so if you're worried about this you should be mixing your coins adequately and also making privacy your number one priority. Just because the same site is selling dodgy stuff doesn't mean your coins will be mixed with dirty ones either. As far as I'm aware money goes to some sort of third party escrow agent so it's not like Silk road where they would hold onyo your coins.