faroot: Well, but also there are a number of ROM-based games already sold on Steam, and for most of them user reviews and guides strongly recommend using a third party emulator for better results.
For instance, if I recall correctly, Sega Mega Drive & Genesis Classics (Sonic, Toejam etc.), Atari Vault (asteroids, breakout etc), etc.
Whenever one buys a game that comes with ROM that is usable with a third party emulator, as far as I know, there is no DRM to be concerned about.
I realize I didn't write it all out but my "what you already have" was meant as a blanket statement toward games purchased or otherwise - sold through Steam or any other storefront or acquired through however you might have acquired them (I'm not the fuzz, I don't judge).
The remainder of the response was on point with what was asked and that's if, beyond the normal use of Retroarch, would the people behind it be able to sell ROMs and as someone from their collective stated - always a possibility if they can work out the details.
For me personally, I wouldn't hold out hope for this to mean some gigantic wave of ROMs from different companies is suddenly going wash up for sale on Steam or anywhere else. At best, this is Retroarch trying to expand their brand with the future adoption of Steam features (again, all this is outlined by representatives for Retroarch in the Steam forums).
It would certainly be nice to see companies get on board and start selling ROMs of titles though.