If one has high quality and relatively rare material to sell on
ebay, I suppose one could to pretty well. I don't; I just sell duplicates from my collection and some stuff I
inherited. I find it pretty iffy. I've had no luck selling my father's FDCs, nor his plate blocks. Still, I find it rather fun, as long as you're not doing it ti put bread on the table.
Good luck with your efforts.
Don