scamers aside to lessen disputes I've come up with the below:
lockout the sales section for non members so they cant even see it, make a minimum post rule to be able to post to the sales. ( i guess this is where your fee could come in)
make a template guideline on the what each post must contain like pictures, location, standardise the selling language
my pet hate is "brand new, only used 5 times
"
in the sales pictures make a rule so that you have to show your username / todays date on a piece of paper next to the item your selling proving you own it.
maybe put the feedback within the listing,
just a few ideas