Before you purchase or list an item...you must think about your projected profit margin on a piece.
There are some helpful calculators on the web, that help you figure your costs that Ryan Olbe has developed. And he has a reserve calculator that helps you reach a projected profit:
Written By: Ryan Olbe
Etsy Fee Calculator
He also offers a eBay calculator:
The other way to look at prices is deciding the market price or what someone is willing to pay to a piece of vintage jewelry. When I am putting a piece up, I search Esty for a piece with the same search words. I then think about condition and positive and negative differences. I come up with a market price and set my price alittle below that. I do that because I am looking for jewelry to sell fast. Other sellers don't mind waiting for 10-20% higher profits. I am of the higher volume at lower profits selling philosophy school.
When I use eBay auctions and I determine the market price in a different manner. I use the same search words to come up with similar pieces and then search under completed listing (on left hand column). This shows what recently sold and for how much and what did not sell. I don't find the "What It's Worth" search button that appears before you list your item under selling helpful. If you want to do in depth research on eBay prices, Hammertap offers ten days free use:
I used it and didn't find any vintage jewelry prices that surprising. But I did find out that I didn't need it.
eBay prices and Etsy prices are different. You need to experiment to find how the different markets work.