I have sold on E Bay and made a good profit from my works as self representing artist.
A rule of thumb for me is to do very quick paintings maybe and hour to do them and then stick them on as an experiment to see how it goes.
If you try to sell work that has only taken an hour or less/more it does not really matter too much if they go cheap as it is purely an experiment.
The last painting I sold went for a great price as it only took me 45 minutes to paint. Dont forget to get an idea on your postage costs before listing your works as some do under estimate the cost finding they have to pay towards it.
I hope this has helped.