My $.02
If you know what you want out of them (or at least a ballpark) post here and see what comes in. No commission, costs or much time investment.
For auction houses here are some thoughts (don't assume that if I left them off they are bad. I wasn't going through every one of them.)
- Heritage - Handles many of the biggest items in the hobby. Recently sold some major jerseys and has some crazy items upcoming
- Goldin - Became much bigger during COVID. More focused on modern and card than vintage GU
- Hunt - I don't really understand them because they have so few images but often get strong prices
- LOTG - Smaller house. Often have a lot of vintage memorabilia (not necessary GU stuff) and Al has always been great to work with and even just talk to. With a smaller house, it may get more attention.
- Grey Flannel - Focused specifically on GU. Have not compared prices
I would reach out to these and any others you are considering. Get an idea of what commission structure, how it will be featured (full-page in catalog etc), auction estimates (although take this with a grain of salt), when it will be listed etc. From there you can negotiate on the details and make a decision.
There's also always eBay which does not take much time. With Powersellers you can get fees pretty low especially on more high-end items.