I think the fees average out about 15%, plus whatever sales tax applies.
I usually run a 10-day auction, and price at or below the lowest price for comparable items.
Buy It Now price has to be 30% more than listing price.
A separate bank account works fine for the managed payment thing.
Some stuff sells pretty quickly, some takes for-freakin'-ever, depending on who's looking for the item you're selling.
Example: bunches of '60s-'80s new car brochures I've listed: the weird stuff sold quickly, yet the Jeep/AMC, Plymouth, and FoMoCo stuff languishes...
FB Marketplace & CL are only better for me in that there aren't the Ebay fees; but worse as far as "buyers" who actually show up or are a pain to deal with.