The cease and desist is NOT about the app. It is about parasitism of the brand.
The use of the API is secondary, but just a part of a proof in a dispute based on the claim of a brand that you are parasiting their immaterial actif called BRAND.
But you don't seem to know how french law works:
* the «raison sociale» of a company has not to be different from another company (ex: Mont Blanc = pen + desert + city), it has to be considered not too close on the naming and on the «secteur d'activité» to be considered parasitism. When you received your kit for funding your society (for instance @chambre de commerce), it is written black on white how the naming of a society should be done. Any brain dead accountant like mine even told me it is wise when choosing a «raison sociale» to first pay a (way too expensive) check into the INPI DB.
* a brand at the opposite of a software is not protected at his divulgation, but at its «territorial» registering (hence INPI and strong advice to make a search at the creation that given your SIRET no company has a name close to yours because else, parasitisme is almost already proven)
* the added presumption of an act are enough to be a proof (faisceau de présomption vaut preuve)
* AND you are dumb enough to make money in the sillon of leboncoin.
Constituting EXACTLY what the french law call parasitism.
I don't support them, but you are legally dumb or dishonest.
I would have respected you if :
* either you had been smart (making money from leboncoin, but registering the brand in a country where leboncoin had not registered (tuvalu? trinidad & tobago)) (remember the territorial part of the brand)
* either you were saying : I know I broke the law, but I think infringing (c) law is stupid because ....
BTW brand are covered by an international convention, so it would have been the same if leboncoin was german, US, or russian.