Few tips:
look out for free listing weekends, when listing fees are £0 for ordinary auctions. Try to avoid paying listing fees if at all possible. Today happens to be free listing!
Ebay takes 10% of sale price, plus 3.4% of sale+postage price if you are paid by Paypal. + 20p as set Paypal fee. My rule of thumb is to call all that 15% of the sale price. Ebay gets 20% if you pay listing fees or for other listing extras.
Weigh the item and plan to add on at least 10% weight for packaging unless it's something you could send in just a tough shopping or bin bag, or it's particularly fragile to pack.
Items under 750g can usually go for £2.20 Royal Mail second class.
750-999g, best price is usually Royal Mail 2nd class at £3.50.
1-5kg best price is usually Collect Plus at £4.99, look at their website for strict rules, though, about sizing and packaging. Might even manage to send for a mere £3.99.
1-2 kg next best price (assuming item is too awkward for CollectPlus network) is usually Royal Mail at £5.30. But if you live near a myHermes shop they can undercut that.
Above 5kg your best bet is to use the courier web sites to get quotes for mainland delivery. These prices are not fixed long term, so I would add on £1-£1.50 in case the deal isn't available when you actually need to book. Ebay allows you to add on a modest handling fee to postage. Make it clear in the listing that the courier price is only for mainland UK delivery and price will cost more to other places.