Perhaps price the items at what you would be willing to pay for them.
Watch the ads here and see which ones seem to sell quickly.
Besides the actual postage (and delivery confirmation and/or insurance) other shipping costs include the packing materials - envelopes, tape, bubble wrap, plastic bags - whatever you use - and remember to allow for time and effort put into packaging and delivering items to the post office.
If you are planning to sell a lot of items, I think it's worth buying a fairly accurate scale - that way you can give the buyers a fairly close estimate of what the postage will be. (Weigh the item with envelope/box/packing)
One can get priority mail boxes and envelopes "free" from USPS - but that is not always the most economical way to send items.