You should try to sell it for much more.
OR
If you think for some reason, you can't do this yourself, then I would suggest the following:
Tell me what your highest offer has been for instance it may have been $4000 and then I can have a go to try to sell it for more for you. Any amount that I sell it higher than what you've been offered so far, we would split the difference 40-60, I'll take 60%.
Let's say I get lucky and sell it for $4500, then I would take $300 of that:
(4500 - 4000)x 0.6 = 500x.6 = $300 to me and an additional $200 to you.
My commision would work out to be 6.6% of the total figure!