One of the ways of becoming a seller with a good reputation is not to over-exaggerate, inflate or simply guess costs. There is an easy way to find these out - by using Royal Mail's website - www.royalmail.co.uk. Armed with a set of scales, you will not only be able to reasonably estimate the cost of mailing items within the UK, but also internationally, a great way to increase your sales, particularly if you have a Paypal account (please see my other guide in relation to this).
You can also do your bit for the environment (and your pocket!), by recycling packaging you receive yourself. I not only sell but do a great deal of buying too. By keeping boxes, bubble wrap etc., aside from sticky tape and some parcel paper, your outgoings in this regard stay at an absolute minimum, and this is a great point to add in the item description so potential bidders know they aren't going to be ripped off.