As a frequent seller I would say just one thing, "Read the Listing", nearly all bad feedback I have ever had has been due the buyer not reading the listing, things like:
"The M4 bolts in the listing are to large for my 3mm holes"
"the 3/8 mamod valve did not fit my TE which only has a 1/4 thread"
"I expected it to be larger (come sooner) (be cheaper) (etc)"
"I think it should be green (a red based SEL)"
all actual feedback I have received over the years