germany
They cannot. Not in your case of a private person selling their private purchase to another private person. It would be a big pandoras box otherwise. What if you gift the item to your friend and they put it on sale? Your friend never had any contract with the seller at all.
Commercial reselling can be restricted in the contract. And that is valid. You can find a rather lengthy explanation of it here: BGH 2008 (AZ I ZR 74/06). It is actually about commercial reselling, but shows what is and isn't commercial reselling.
But you cannot restrict a private person to resell their property once they bought it.
If a ticket is personalized for example, the seller has to offer a way to change that personalization, given that the old and new owner agree and take part in that process. This is supposed to keep so called "scalping" down and make it harder for commercial platforms to be a middle man.
So if you sell this on your private ebay account, once, probably for less than the original price, that is totally legal. If you use a commercial account, sell more than one regularly, or make huge profits, that are indicators that you are doing it "commercially" and you may get sued.
I'm not entirely sure what your options are regarding your good connection to the seller. They aren't required by law to sell you something, so... insisting on your right to do this might seriously jeopardize your "good relationship" and they might take all legal steps available to them, which may include never selling you anything again.