It doesn't seem weird, but the coin dealer is obviously a high risk account for two main reasons...
Access to currency and precious metals through "less than legal" means.
Dealers require a specialized knowledge of coin value that is hard to determine.
So in my opinion a money laundering situation would be...
"Coin dealer" opens a shop... and buys and sells "coins" for cash. When in reality the value of the coins being bought and sold is hard to determine, as is the source of the cash used to purchase the coins that the "dealer" sells. Also as in the situation that your are describing they could be providing the cash to the person who wrote them the check... who is then giving them the "clean check" to launder the money.
In either case there isn't much your institution can do about it besides monitor and follow your gut. If you think this whole thing is suspicious, file, its not defensive if you legitimately feel like this is money laundering.
In life, there is a lot less that could get better and a lot more that could get worse.
MBA Fin/MBS HR
My views only!