Ditto to what the previous poster said.
You really have to sit and work stuff out, prices, suppliers etc... I tell you I made some stupid mistakes when I started a year ago, lost money, made some money and I still make daft purchases (not so many now, thank God) that I really don't have a hope in hell of selling!
I tell you, I still have 2 pairs of designer shoes that I bought at Xmas on whim and I don't have a clue how or where to sell them on...
PS, one tip, read forums like these and the eBay forums. I learnt stacks about how to spot scams & ripoffs and just how to do things, like the best times to list!