So if I understand properly, the "correct" way of doing such transactions (in a way that it's not too hard for both parties), is really to ask the seller to send the money to one of my address, and then forward the amount to the seller.
Thanks for your help.
Not necessarily.
You can also ask the seller for their xpub key and use it to derive a fresh payment address for each individual sale. This way the seller retains full control over their private keys and you can both generate and monitor payment addresses.
Essentially you'd have a watch-only wallet while the seller stays in full control of their funds.