I did a few years ago. It was a VBN Mutant Buzz from memory. I guess somebody just didn’t want it, couldn’t be bothered trading it or just didn’t know it was unconverted.
I would start by employing a good customer support team to fix all those issues first. I'd look towards the Neopets community for old players that actually have an idea about the game to help with what actually needs to be done in order to fix the site.
Stamp is more profit but also more crowded. I've been making a lot of NP in Magic now that you can sell most of them in your shop. The hard part is selling the 1mil+ ones because you have to either advertise or be actually logged in to accept offers