If a wrestler wanted to buy his own product, will it come out his check or does he pay for it personally?

Logically he would pay for it the same way everybody else does there and then with money over the counter or a plastic card.

Only difference is that wrestler would receive a percentage of what he paid for it inside his royalty check 3 month later.

To I think it’s fair to say that for eg the walking merchandise stand John Cena doesn’t pay with any of his money every week on Raw before the show starts.

You’ll have to ask a Wrestler that. I don’t know and have never even thought about asking them.