Fortunately rules exist to regulate transactions, including and especially on the Internet!
Whether for the Business to Consumer, or Consumer to Consumer. But again, ‘the law is a complex matter’.
For example, a particular seller can be (under French law in any case) legally qualified as a ‘merchant’ according to the regularity of his sales activity. Thus, regular acts of sale on a merchant site or even a simple connection site, can make an individual a merchant of fact (within the meaning of the Code of Commerce). Various others criteria can support this... And this is pretty good for protecting transactions.
Not knowing the context of this transaction in particular, the site where it took place, or even its amount and the profit made, and not being a specialized jurist, I will stop here my advice. After all, we are here only trying to bring some leads @lazarusgordon
PS: I have been ordering various gears for years in France, Germany, Spain, etc. No problem to report with La Poste on my side.
'On ne devrait jamais quitter Montauban' (Fernand Naudin).