I'm in a similar situation with a Graupner Optimist (not a dinghy but a 1960s One-Ton Cup yacht) I bought recently on eBay. It was described clearly as a model in need of repair + restoration - the mast + rudder were broken and the rigging, fittings etc. were in a mess. However, the hull and keel shown in the photos were clearly intact + whole.
When I received the model in the post, the lead keel had snapped off leaving a large hole in the bottom of the ABS hull (see photo) . This turns a straightforward restoration project into a lengthy and difficult repair job

This had definitely happened "in transit" - the boat was packed in a large cardboard box (significantly larger than the hull) surrounded with foam chips, so there was nothing to stop it from moving about when dropped.
The seller appears (from their feedback + other items for sale) to be a dealer/semi-professional seller, but they specialise in model railways and presumably have little experience with packaging this sort of item. The boat was not (unlike Mi Amigo's purchase) packaged too lightly - it was in a sturdy cardboard box with enough foam chips to fill a black rubbish sack - just inappropriately (there needed to be something - e.g foam blocks, cardboard pieces - to securely wedge the hull inside the box and stop it moving around). It doesnt look like the seller wasn't deliberately negligent or careless (they have 100% feedback with nearly 10,000 transactions), hence I'm reluctant to just leave negative feedback.
Another factor is that I won the auction at a ridiculously low price (£35 including postage). I suspect this didn't bother the seller too much as it isn't their main area of specialisation (maybe they got it as part of a collection/job lot) and with thousands of previous sales they're no doubt used to items sometimes selling for less than expected. However, it does seem (to me at least) a bit unfair to expect even a partial refund when I paid barely more for the model than it would have cost to buy the lead for the keel!
I haven't yet contacted the seller or left feedback; any suggestions as to what I should do? I'm inclined to leave positive feedback along the lines of "damaged in post, but still a bargain" and e-mail the seller about the damage (maybe with suggestions on packaging), so that they can avoid a similar occurence in future.
As for what I'm going to do about the keel damage, I'm planning to shape a wooden block to fit the bottom of the hull where the keel joins, which will both plug the hole and provide a rigid mount for the lead keel to be attached to. The keel attachment seems to be a weak point of many Graupner ABS-hulled yachts - in most build logs I've seen, the builder has added an additional wood plate or bulkhead inside the hull for the keel bolt to attach to - and I'd probably have added some sort of reinforcement here anyway, but it would certainly have been a lot less work if the hull + keel were intact!