I have seen a new online paper money auction platform called 17 Auctions. I have no clue (how good/bad these guys are) but there it is (& they post that their buyer's premium is 10% which is pretty reasonable compared to 20% for the big houses). I think their 1st auction is in October.
The problem I have with HA, Stacks & Bowers, Spinks & Sons, Noonans (& any big auction house) is that they have high buyer premiums, S&H, plus will charge you to use your VISA. In addition to this, they're a red flag to your customs so if you're not bidding from the US (or UK for Spinks/Noonans) you will be charged duty plus a service charge for their carrier (like FedEx) to pay for your duty. It all quickly adds up! Read their “Terms & Conditions” before you bid!
I have dedicated a Money Market page to those interested in finding other ways to acquire banknotes on my Notaphilyc Cutlure site. Check this page out if you have the time (& thanks for dropping by). There may be other sites I'm forgetting (& coin resources) too.