There's no problem (IMHO) if you sell them to another collector or to a dealer.
I have a couple of guns in my collection that I could be persuaded to sell if a friend wanted or needed one. But I didn't buy them for that purpose. Truth be told, the whole concept of *selling* a gun is foreign to me.
Some have said I can only resell at cost or less for this type of activity. $840/8 = $105.
If you have to apply a formula to it, any formula, you're probably over the line.