Spirch, here is honestly what I do
Look up original selling price of both knife and sheath
Base price of that.
maybe add 10%
If used but not abused or modified (ghetto satin, stripped, regrind and such) you should get at least base price or what you paid.
see how the market is, That could mean a lot, a original satin Jack would be worth more than a Battle Grade Boss Jack
If those were mine I would sell at
AK BG w/cool Buy Brown sheath no Tek-lok = 575.00 or 500.00 for a quick sale
Rattle Hawk w/cool Buy Brown sheath = 415.00 or 350.00 quick
TG-M (It is INFI) w/Buy Brown sheath No Tek-lok = 450.00 or 400.00 quick
Hope that helps.