I don't think there is as many criminals on stamp as you think. If you were a criminal where would you sell?
BTC-E and local bitcoins are probably best. I dont think many criminals are hoarding much coin either as they have expenses to pay, unless they got lucky on the run up. I suspect most criminals aren't into BTC for the technology but rather the means of payment. The last 6 months has probably had most of them running for the hills