I hear ya,
I thought about the legalities of MAME, and I'm the last one to want to step on someone's toes ... so I thought about this ...
I suppose just the sell the PC, ( without sounding to much like a smart alec ) in an oversized case.

I see some the vendors selling a 3&1 , or 6&1, they even have a 24&1 Arcade Classics , which is nothing more than a MAME in its own right. So perhaps, a GamingPC ... 24&1 unit.
....
I remember this guy at the FleaMarket once who was selling PiratedSoftware ... off all things to sell at the fleamarket ..
but anyway , when I asked him how he could sell that here, or even make money from it ..
his response was .... " I'm not selling the software ... I'm selling the CD , which just happens to have some programs on it"
