Because you are not supposed to open a HD, the best way to test its real capacity is to use a software program like h2testw.

As springy said, you could still try if you know how to put it back together, but normally the best way to test a HD is to use h2testw (or any similar software).

Sadly, the more buyers buy from these sellers, the more they support them, not only in terms of money, but also of fake reputation because eBay gives sellers who sell a lot nice titles like "top-rated" even if they have horrible feedback, or "one of eBay's most reputable sellers".

Additionally, many naïve buyers just connect the HD to their computer, think the capacity is fine and give the seller positive fb, and as fb cannot be changed on eBay, when they find out the capacity is not real  after adding more files, it is too late (not only for the fb, but often also for a case).