I had this recently on my Forgeworld timber bridge which I finally assembled and painted after it had sat in a box for 10 years!
Like an idiot, I forgot to scrub it with detergent, and then I used a Halfords spray paint on it that was not a genuine primer. It works fine as a primer on plastic and metal figures and models, but resin with some residue on it, even after 10 years - evidently not!
Most of it was okay, but there was one area where the paint kept flaking off. In the end I just had to scrub all the paint off that whole area, use a bit of neat detergent on a toothbrush, wash it, mask the rest of the model, respray it with proper primer, and then repaint it. That seemed to cure it.
I previously tried painting over the flaking areas with enamel, and with PVA. It still kept flaking.
I would say your seller is perhaps not telling the whole truth when he says he scrubbed it and used car primer. Either that, or the manufacturer used some particularly fearsome release agent on the mould, and those patches of residue proved impervious to scrubbing with detergent and priming. But that would be quite extreme.
I sympathise. It’s a major pain in the butt to make good paint flaking off resin