Well, I'd say simply having to register to be able to sell BTC would be an issue. FinCEN, as much as it has a lot of bad things at its disposal that it has not, in fact, done to date, has a fairly lengthy history of creating regulatory uncertainty and then suddenly prosecuting some practice that has been, to date, tolerated. While I am not a full-bore anti-government loon by any stretch of the imagination, and in fact, FinCEN's public official statements to date have been actually pretty welcome and reasonable, there is still an environment of regulatory uncertainty.
AFAIK, FinCEN has never stated that you have to register to be able to sell BTC.
There have been some cases where people were charged with money laundering or running an unlicensed money transmitting business, and those cases involved selling BTC, but it wasn't alleged that selling BTC is illegal by itself.
There's a lot of nasty things that FinCEN potentially could do, but until they actually try it, there's not much basis to sue them.