I had Poulsn sell a custom .257 Roberts single shot for me about 8 years ago. Gave it to them at the NE sxs and filled out the required paperwork. I believe they kept 20 percent from me and the BP was 22 percent. They estimated it would sell for between 5 and 7 K. It sold for 5 K. I had 5K in it, and got 4K out of it. It was in their fancy glossy catalog with great pictures. You win some and you loose some.
For collectable guns I have had good luck with John Lowenstein. I think he runs a great auction and has a great facility in Maryland to run an auction. Just my 2 cents.