It is possible, only for some shops. If you run UGF for all your tanks, how can you have any media, aside from usually horribly messy gravel, to offer a new customer? There aren't going to be many customers that would like to add a ag of filthy gravel to the filter, because you know, a filter must be spotlessly clean for it to work

(think back to when you were a newbie with little clue about what was going on...) most would just either walk away without it, or stick it on top of the rest of their gravel, where it will do little to nothing...
For a centralised system, you can easily incorporate a mature media section. Just set-up 1/3 of the sump as a dedicated "mature media sales" section, where you go through in order pulling and replacing the media bit by bit as required, such that the media sold has been in for a month or more. You also get relatively clean media and the ability to offer different media types. You are however prone to introducing disease into the tank via this method, as UV comes after the filtration bit, to avoid clogging the UV with rubbish.
It's a good idea, but how can you make it practicle and safe for the customer?
All the best
Rabbut