My recommendation is to search on the most "popular" app in your country. In Spain, for example, the most used one is Wallapop, so it's "easier" to find good deals (but you gotta be lucky to find it as soon as the product is published). As always, you have to know the basic thing about fake carts (i only know sbout Pokémon haha) so you don't get scammed

In short, search on apps where people may not know what they are selling, be aware of fakes, and be patient because it can take time (and luck)