Hi there, I have been buying/selling on eBay for a few years now, I tend to use the Royal Mail website to get prices for posting, I weigh the item in question on a set of kitchen scales then look up how much it will cost on there, I haven't gone wrong yet, if something works out at say �1.28 (for example) ask for �1.75 that then allows for packing costs as well, if selling something like a mobile phone then offer recorded delivery as an option, don't forget this costs more, always get proof of posting in case something goes astray.
Have fun.