The one thing you shouldn't do on Ebay is charge postage separately because the Govt force Ebay to charge VAT on postage, you are much better putting the cost of postage in the price of the item, and offering free postage, like most of the commercial firms do .on the £4.20 quoted the VAT is 84p, if included in the sale price you would be charged 41p, and remember Ebay now charge VAT on their charges so the 10% net works out at about 12% gross, depending on what you are selling.(there are different rates charged)
Always watch for their offers, were you pay a flat fee for your sale item.Usually £1.
I have just bought an Item off Ebay, price was £1250, paid by Paypal, seller rang me as only £1208 went into his account, he wasn't aware of the Paypal charge, I pointed out that Ebay would be taking their slice as well, but he had posted it on an offer where he paid no fees for selling, which was a bit of relief to me.
One other point about selling on Ebay, if you put your contact details in your ad, they will charge you for selling it even if it doesn't sell, or if you are lucky they will give you a warning not to do it again.(personal experience)