They are a very large coin operation and have pretty good brand reputation. However, I have never bought anything from them that I know of.
I haven't been a customer, but have interacted with them. They seem professional. There are members here who work for them @ModernCoinMart is one of their accounts.
Very fast and professional, though they have a somewhat limited selection of coins for sale that have all been struck within the last 24 months for the most part.
I have bought from them off eBay and bullion directly from them and they ship within a few days and never any problems. Received a gold guilted coin this year that had spots on the gold. They replaced it, no hassles.
since MCM's main business is top graded coins I always wondered if the others they sell are the ones that don't make the grade. they have do do something with them.
They used to actually sell modern coins besides NCLT. Now they are essentially modern world slabbed bullion dealers. I have never had any problems with them, but I was sad when they adopted their current limited inventory.