The german word for that is 'Bauernfängerei'
Which is clear, if they buy it from you, then sell it to a dealer who then sells it to the next owner. These are two intermediate steps where someone needs to make their cut.
You as the seller would get the most if you sold directly to the next owner, but that is much more hassle for yourself. You have to make appointments with people who want to see the car, test drive it, ask a lot of questions, and then try to bargain for a low-low price.
If you sell to one of their sites, you get less money but it's a very simple and quick transaction.
Your translation is for BauernFängerEi ("farmer catching egg", which doesn't make too much sense), while this is BauernFängerei ("farmer catching/trapping"). "...erei" is similar to "...ery" in English.
I guess the saying comes from the prejudice that farmers are not the smartest people, and therefore people trying to catch/trap/trick/prey on stupid people are called "farmer/peasant catchers".