Yeah, this is extremely infuriating. I inquired about this from eBay, and they told me that this is due to import restrictions to certain countries. They were unable to tell me what this import restriction in this case is, and pointed me to contact customs office in my country (Finland). I told them I had imported quite a number of watches over the years, and there are no restrictions to importing 2nd hand watches. This did not convince them, and I sort of understand that, as I am not a customs officer, even though I think the number of countries banning the import of old watches must be rather close to zero.. Then I pointed out that they should at least be able to tell me on what basis they blocked the bid, as it was their system that actually did so, based on the information in the listing. They told me that they can't tell me this because it is up to customs, told me once again to contact the customs office in Finland. After insisting on a proper answer, they claimed that they had answered my question in full detail already and that was it! I just quit at this point and have not bought anything from eBay since.
Oh, and this was not anything to do with alligator skin straps etc., or possibly fake Rolexes, these were vintage Seikos on metal bracelets (or without bracelet), and not very expensive ones at that.