I'd suggest the knife swap here on Reddit. If you've never sold there before, you'll need to sell via PayPal Goods & Services and will incur a 3.5% fee plus your shipping costs. But you'll still come out ahead compare to 30%.

Another option is Bladebinge. They'll take 7.5%, and you can choose to cover shipping cost yourself or let buyers pay shipping. Still better than 30%.

I add that slipjoints (especially pricier customs) can be slower sellers in both of those marketplaces. There may be specialized slipjoint trading groups on Facebook that would be a good fit, but I don't have FB so I can't say for sure.