On one hand, there are pictures of the guns before and after painting on the gun's sale page along with a mention that they are painted flat black. To any alert shopper, it should be quite clear from the pictures that there's something unusual about the guns. Missing fire selector markings, for one.
On the other hand, I think Sam could've been slightly more clear by using a bigger picture or more descriptive text. Even better, instead of spray painting them, he could've stripped off the spray paint. That's a lot of investment on his part depending on the size of his stock for what are otherwise rather cheap guns.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."