Iospirit were handling registrations until
March this year. Their "Goodbye, Amiga Market" message suggests they are passing the baton on to
ibrowse dev team for registration and support. Unfortunately it looks like the ibrowse dev team have yet to update their website to reflect this, as
this FAQ shows they are still pointing people back towards the iospirit website.
A banner at the top of the page does state, however that
IBrowse 2.4 is temporarily unavailable for purchase. Apologies for any inconvenience.
I'd try, as Jeff suggested, contacting the
ibrowse mailing lists to see when registration will become a possibility once more.
Also, iirc, the iospirit website only allows you to download the keyfile for a limited time after registration, so as futaura pointed out the seller really should have provided you with the keyfile, as you have no way to acquire a genuine one yourself.