I'd assume that Marcone's ability to use magic is something he negotiated for specifically, before he fully took up Namshiel's coin. Harry showed him over and over what a ridiculous force multiplier magic was, and then the fomor showed him how inadequate his elaborate and expensive defenses were.
The scary question is -- what did it take for Marcone to get that? We know that there's a way to "eat" the magical talent of other wizards -- sort of like Harry did with Kravos in Grave Peril. I'd imagine Namshiel knows the details of the "horrible" way to make that work -- a sort of one-soul-at-a-time Dark Hallow. Perhaps some of the lesser talents who vanished over the last few years weren't taken by the fomor.
Re: the Eye...
When you consider what it takes to use it (physically and emotionally), it's not actually that useful to anyone sane and human-sized. And it puts a huge target on the back of anyone known to have it.
But yeah, I'd imagine Marcone could have taken it and then auctioned it off to the magical community, or just sold it to Vadderung for all kinds of things -- like personal magic instruction.