ann,you weight your item on reliable kitchen scales, add a bit extra weight to allow for packaging, then look at this chart here for UK postage ftp://ftp.royalmail.com/Downloads/public/ctf/
rm/RMLT17_UK_Wall_Chart2.pdf
There's a separate chart if you want to add postage options for overseas. Don't forget to add a bit extra for your brown paper, tape etc., and if you have to use petrol to get to the PO.
If something costs more to mail than the postage ceiling allowed for eBay in some topics, just add a bit extra to the starting price of the item, to cover it.
Btw on free listing days, it's only the first photo that you get free - if you add more than one photo, you pay something like 12p extra per photo.
Take your photos during the next couple of weeks - I find free listing weekends come up about every three weeks at the moment.
I share your frustration about keeping things - my house is stuffed (and I mean stuffed) with stuff, and I find it very hard to get rid of it, but eBay is helping - what I sell on eBay goes to spending money for our holidays.