OK, but there is another danger in such transaction - "Trusted forum members" may get into trouble if they offer such service to someone not that trusted who want to launder some bad money, this is especially dangerous with Monero and other anonymous coins.
Doesn't that apply to all sorts of different economical activities on this forum?
I can ask for a loan in Bitcoin and offer 'dirty' altcoins as collateral from where I simply default, which is basically similar to a trade for the one looking to get rid of his tainted coins. What happens with these coins? They get converted to Bitcoin. I can use 'dirty' (most sellers are willing to accept XMR as well) coins to buy a loaded physical Bitcoin, which is also similar to a trade. What happens with these coins? They get converted to Bitcoin.
In the end, it's up to the trusted forum member or seller to look at everything on a case per case basis, and from there decide to do business with a person or not.