Greedy boy beat me to it (yet again lol!), he's right it's 3.4% + 20p as this will be taken off your receiving payments... Another thing to consider is eBay fees if you plan on selling on there too, i'm still playing around with getting it right as it depends what your selling price is - i just work on an average of 7.5% plus promo fees (i.e: subtitle, bold, gallery e.t.c...)
Hope this helps & happy trading
Steve