I'd personally be very wary of buying what is really just a serial number from eBay. For sure, it could be a full retail serial number that works fine in a current version - but who's to know whether or not the vendor has sold the same number several times under several different eBay IDs? If so, and Ahead realise it's a serial number that's got "into the wild", they could always block it in a later version. There's also the possibility that you'll pay your money, get the serial number, and find that it isn't a full retail serial number after all.
The situation is a little different with programs where the serial number is always bought on a recognised card, label, CD case or whatever - but even they could be fake.
If you've got a bundled copy of Nero 5.5, you're entitled to buy an upgrade online from nero.com - that's what I'd do under the circumstances.