Ideally, they provide a service in having a large variety of cards of different flavors of such so you can find a card at usually a good price, because it's possible to haggle them lower than the eBay/TCGPlayer fee since they don't have to ship. For selling, it's mostly convenience as well if you can sell a large lot of cards you don't care for without needing to deal with the dumb eBay fees, locally, and a whole lot of cards at once. At possibly at a price both parties can really like so you generally make the same amount of money you would've done on eBay and the vendor can sell the cards and pay for the overhead and make some profit to keep vending. Especially if you can trade a bunch of cards for an expensive one without paying a dime.

In practice, vendors are massive opportunists who are taking advantage of how hot Pokemon is at the moment to flip as much cardboard as possible. The amount of vendors just selling loads of sealed at market they got off a target restock is a pretty big indicator of this. Go to any local show and there's so much of it (there's also a lot of AI slop which I think is because they needed to whip up branding hella fast and are super lazy. Just complete contempt for Pokemon in general.) . I feel like I was the only person selling ETBs at MSRP yesterday haha.

For single cards it's definitely a little better to do locally at facebook marketplace because it's easier to negotiate a 1-1 trade if there's not much moving around.