You should contact the eBay seller and ask them to send you the corresponding IBrowse2Key file. You don't mention the eBay listing URL, so it's hard to comment, but if you were specifically sold a registered copy of IBrowse then that should have included a key. Presumably the serial number has already been used to register that copy and retrieve the key, so even if the HiSoft site was still up, you wouldn't have been able to re-register it anyway.
If the seller won't provide a key, then they've ripped you off and essentially sold you a demo version. IBrowse parted ways with HiSoft 4-5 years ago.
