You don't really make money, but try to cost up everything carefully so you're not out of pocket. To keep track mentally, I tend to list the item for start price I can live with and then try to load up the postage with all the fees side of things. Suppose I really want minimum £1 for something that will cost £2.20 with RM to post (small packet price 2nd class 100-750g = £2.20).
(there are online calculators to help you do this part)
So if I charge 2.69 for postage that covers
£2.20 for postage +
20p fixed Paypal fee +
12p (3.4% of (2.69+99p=3.68) = 13p)
- 10p (Ebay's final valuation fee from the auction itself, 10% of 99p = 10p)
= 2.62 total
So buyer pays £3.68, RM gets £2.20, and I get a mahoosive £1.06.
Loading postage to cover fees is a little risky, plenty of people will jump & down screeching how outrageous it is to charge 49p over Royal Mail cost and that private sellers on Ebay are pondscum have no right to charge anything for their time or packaging. Ebay Corp's actual guidance on the matter is pretty fuzzy.
So you could start the auction for £1.49 instead to try to avoid low DSRs for postage rates, but then you have to wait until FLW or take a hit on listing fees. And some people give low ratings on DSRs no matter what you charge, so I can't see point of pandering to folk who have silly expectations.
I think you pay a fixed small fee if you sell stuff on Gumtree. Good alternative for expensive collect-only items in high demand areas. Local freeads, too.