[underwater]

It's not really redundant. Legitimate users use redirectors like bit.ly all the time, so you can't blacklist them. If you're leaving such a big hole in your system then spammers will work around it in next to no time.

Etsy are big enough that it is worth the spammers time to do so. Once you get reach a certain size you can't just say "the user should be careful". Scammers and spammers will hammer at you because they know the numbers make it worth the effort. Users won't understand what's happening; they will have a bad experience and they will blame your product.