The removal of the cape redemption page has opened a window for scamming
The page for players to use their cape codes was removed some time ago, for reasons I presume you considered valid. However, the endpoint API for this service is still functional and is used by trusted members of the account community to redeem codes.
This is the method used: send a POST request to https://api.mojang.com/user/profile/<uuid>/capetokens/code/<capecode> with the "Authorization: Bearer <access token>" header
Due to the complexity and 'protected' nature of this method, regular players are unaware that codes are still valid, or require assistance in using them.
This is used maliciously by scammers within the community, mass emailing players asking for codes on the grounds that they no longer work, they'll redeem them for you, etc etc.
I was prompted to make this post by the fact a 2012 code was recently scammed and introduced to the market, these retail for $2,000+ so you can understand how it's upsetting for me to see that happen.
My suggestion is bringing back the page, after all you'll need it back for Minecraft Festival. I'm sure you've got valid reasoning not to, but I'd heavily suggest attaching the Minecraft Festival capes to accounts, similarly to 2011.
I don't think this is possible, but if it is, I'd recommend doing it. In any case, people are already planning to buy dozens of tickets to collect the cape codes and sell them for a profit. Even Discord servers exist, created by planned attendees, to redeem a code for you as they need to be scanned in person to be valid.
I'm not full of suggestions here, and I hardly know why things are how they are, I'm just reporting the current state of the cape market so that you can review the information and hopefully make a sensible call.
I hope this is useful