in one of the vive canals, I picked up this evil holly knife.
It's listed value is 10,000. when I try to sell it, I offer about
5,800. I can't find anyone with more than a thousand. who can I sell
this too, or where.

In Caldera.. there is a house full of orcs.. in the 1st floor (not ground floor) there's a kreeper with 5.000. You can sell it to him.

Or simple to the old way...barter trade.

Trade in your 10K dagger/knife for some expensive junks from the trader. Then sell the items back one by one until you have sold off all of them. This may take a few days because some traders only carry the maximum gold of 1k per day.