Why are you so concerned about including games to sell your cabinet?
The difficulty with selling a cab without games is that the kind of people who would be willing/knowledgeable enough to load games on it and get it all running are the same kind of people who would just build their own cab. When the average joe schmo plunks down $1000+ for an arcade cab, they just want to plug it in and play. I think this is why cabs with xxin1 boards in them seem to sell better than cabs with PCs in them despite the fact that the latter can play way more games, is more customizable, and will likely run the games better.
I think the best way to sell a cab would be to not include Mame or any roms, but make setup as easy as possible. Basically have a nice frontend all set up and tell the buyer where to download Mame and to search for roms on torrent sites for the games they legally own. Give them very clear instructions about where to drop the Mame exe and the roms. Offer to help them with any software issues they may have for free up to 30 days from the date of purchase or something like that. Granted, this is still going to be less appealing to most people than just having a cab pre-loaded with games, but it's legal, and you could try to spin it as, "Don't buy a cab from someone else and let them choose what games you get. Buy from me and pick your own games." This is probably the best you can do without breaking the law.
You may not sell a lot going this route, but seriously, you should be doing this for the love of the hobby anyway. It's a niche market. I'd be surprised if anyone out there could scratch out a decent living selling Mame cabs.