Whether eBay or Basenotes, you would indicate whether you ship only to the U.S. or also internationally. For U.S. destinations, you could indicate a flat rate (say $5, $6, $7) that you think would cover any place you would ship to. If you set an appropriate flat rate, you don't have to worry about figuring exact shipping for each purchase (but you have to eat the cost if actual shipping ends up being more than you asked for). Many sellers offer combined shipping if more than one item is purchased, and you could make it optional for buyers to pay extra for shipping insurance.
When I offer international shipping, I usually look at rates on the USPS website to get a general idea of what to ask for, depending on the country.
If you browse some of the current sale threads on Basenotes, you may get some ideas for how you'd like to sell.
I hope this helps!
A little more:
Prospective buyers will want to know size of the bottle, whether it's edp or edt, if the item is brand new, sealed in box, never tested, tested once, half full, with box, without box, with cap, without cap -- and any other details that you think might be pertinent.