I to have bought and sold boards, fins , leggies and covers from the Matt and Anthony at the Boardroom for many years.
They are surfers,own the shop and also occasionaly SUP and have their finger on the pulse with the prices, which is probably why they have so many choices of boards.
They offer a fair/realistic price, not an over inflated price which you then have to deal down to a fair price.
A few times they have told me this is all my favoutite board is worth, and to try and sell it privately to get more which I have done.
At the end of the day, do your research and negotiate a price that is suitable to both parties.
Everything is negotiable.
If your not happy, walk away as there is alway another board around the corner