I buy and sell vintage Ray Ban sunglasses (especially the aviator style (surprise) and, while it is fun, I just naturally don't want to lose money on it. I list the item with at least a 10% markup, depending on the style and condition... if it doesn't sell... list it again after a while. I've relisted some items 3 or 4 times only to have them sell immediately. I always list with the five day auction since 7 days seems to dissuade some folks who just don't have the patience for that long. I never use "Buy it Now" since it adds to the selling cost, but that's just a personal quirk, I imagine.
Always research and thoroughly understand the additional shipping charges, 'cause, as you've already deduced, they can be significant.
Just bought a pair of 30 year old Ray Ban's from a guy in London... don't know, of course, where he comes up with them, but they are all very high quality for being anywhere between 20 to 40 years old... great to do business with. I watch the other listings on Ebay for guidance on what things are bringing...
Good luck! (How's your Papeete trip coming?)