OK, this is a good direction, but there shouldn't be an "only in case" condition. There should not be any condition that you keep someone's money, just return it if you don't want to accept it in exchanging order.
You are not the SEC, or the FED, nor are you in charge of such things, plus it is a susceptible issue and it is very difficult to decide with certainty whether the funds came directly from some fraud activity or were only part of something illegal in the past.

We already have many instant exchanges and with most of them there is a risk of funds being blocked at some moments, if you would avoid this scenario it would make you stand out from this group.