What concerns me is not multiplicity of accounts - but multiplicity of accounts with DIFFERENT DETAILS, all belonging to one person.
That is, different name (not just username - that's a given), different address, different financial information. And of course different IP so that neither eBay nor PayPal can easily use their "sophisticated" technology to discover the relationship. (Excuse my use of the word "sophisticated" in this context. There is enough irony there to build Titanic Mark II, III, IIIb, IIIc, IV, and V.)
Shyer, your point about 500 IDs is well taken. It is not hard for a banned user to rejoin eBay using different (invented) details. Creating multiple IDs on PayPal is, as mentioned, not quite as easy, but it's certainly not impossible. More than that, it is sufficiently possible that a reasonably average person with reasonably adequate skills - without criminal connections, without being a criminal mastermind - can do it.
Smee, you're right - legitimate users with legitimate non-criminal purposes can have multiple accounts, and as long as the reason for those multiple accounts is indeed non-criminal, it doesn't really matter (except that eBay doesn't distinguish between one user with multiple accounts and many uers with one account, so that their membership information is grossly skewed and non-reflective of reality of use in our society).
But when creating multiple IDs on eBay and PayPal for legitimate use, one doesn't have to a) disguise one's IP (proxy server, false IP setting, etc.), b) create or use a different account holder name, c), give different address information (please correct me if I'm wrong on this; I believe someone did once say a different address had to be entered, but on the other hand I've heard of multiple accounts with the same address), and of course, d), provide different financial information.