Take accurate measurements for clothing you list on eBay. Do NOT make assumptions. Remember, all brands size differently. Trust me, I’d be thrilled if I could walk into a store and automatically know what size to purchase, but it’s always a gamble.
If a prospective buyer asks a question that you cannot answer (e.g. I wear a size 8, do you think a medium will fit me), respond to them by giving approximate measurements for the garment or directing them to the manufacture’s sizing guide.