I cannot ask the sellers to change their receiveing address by themselves, after each sell, this makes no sense.. Imagine there are 3 sales within one minute for example.
Actually, you can.
And it is the best way to accomplish what you are trying to do.
If you are some kind of intermediary (without direct connection to the sellers server via an API etc.), you would request your seller to give you 10k addresses of them (generating them doesn't take more than a few seconds).
Then each time a customer wants to buy something from seller X, you give them an unused address of seller X. Once you hand out this address, regard it as 'used'. Even if the buyer doesn't actually buy something.
By giving each deal (customer Y buys from seller X) an unique address, it is way easier for everyone to check whether the transaction occurred and whether the amount is correct.
Once the address pool is 'low' (e.g. < 1k addresses), you request another 10k addresses.
However, if you as an intermediary are needed and don't want to regularly stay in contact with your sellers regarding addresses, using an xpub is probably the best option.
At least if a lot of sales are happening and refilling address pool would have to be done quite often.
Why exactly do you think it is not a 'good idea' for them to share their xpub with you ?