Um, maybe.
The venue has no obligation to accept phone scans, and a growing number of venues is refusing to allow people to just scan a PDF off their phones rather than have a printed instrument because it is a growing vector for fraud.
Care to name a few with which you have any agreement?
If you consider a marketplace to be (demand side) : (supply side), they already have a backstop in the supply side through price-comparison, and can now differentiate their service from other companies with a solid user-uploaded set of tickets.
I'm a season ticket holder for such a team, but can't go to all the games, so I list quite a few each year, and I really hate that demand for tickets gets unequally spread through the official market and these other markets.
Is there a legal reason why a seat aggregator like SeatGeek doesn't list these official site tickets? Is it just because they don't get a kickback from them? Some other reason?
TicketMaster, the monopoly that they are, can do this with their resale marketplace (of course, the ticket needs to come from them in the first place), but since SeatGeek aggregates, I don't think they have this superpower.
An individual can do an ad hoc validation of a given ticket by trying to create a listing on the official marketplace. You will be able to create a listing only if the ticket is valid and unencumbered (by an existing resale listing).
re:fees, we're charging no fees for user-to-user sales. For tickets listed publicly, we're charging 12% + 3% for credit card (market rate in our industry is 20-30%). That's preliminary – we may move it up or down.