I bet you are exactly right. $10 seems high though. But maybe people will do it. Wonder if the are allowed to sell those. I am sure they wouldn't do anything illegal.
The price sounded high to me to, but then I checked Vudu, and they are selling the HD digital version for $19.99. The regular price for the BR/DVD/Digital is also $19.99 right now. So, someone could look at the $9.99 price and see it as a way to get a digital copy for half price. Personally, I wouldn't buy it as for this particular movie, I'll bet we see the BR at $9.99 for the holiday season.
I think it is legal for them to sell this under the first sale doctrine. It is the same law that allows them to buy retail copies and rent them even without explicit authorization from the copyright holder. In this regard, it allows Redbox to dispose of, including sell, copyright material as they choose. (I am not a lawyer.)