Is it allowed to buy cards from other players at pokemon leauge, or do you have to trade cards?

Leauge night is tonight and thanks to scalpers (well, in my area it's one guy who buys everything up to resell at 3x markup at his stall at a local indoor flea market and on marketplace) I've not been able to buy very many cards and the ones I've ordered don't get here till next week. I don't have a deck built, but I had a plan to try and buy singles from the people there to build some kind of deck.

When I played leauge 10 years ago, they didn't allow people to pay money for singles at leauge, you either had to wait till after and go off property or you could for example, buy a pack of cards and just trade that for the cards you want.

I'm wondering if this is an official leauge rule, or just one my old lgs had.

EDIT: Turns out, it's a real casual bunch, and they just gave me like half the key cards of a deck the noticed I had a good chunk of in my collection :)