I would think that this is a very limited market - you are seeking women with fertility issues (that's only women first off, and only those with fertility issues, which is a smaller number) who also think that certain stones will balance their hormones (another very small percentage) or who believe in saints (a smaller-still percentage of women, even among those who are religious). Then you want to narrow the market even further by looking for women who want to advertise their infertility status by, say, inviting questions about their jewelry. Think about it: You're in line at a grocery store or buying make-up at a counter, and someone says, "That's a really nice necklace. What does it mean?" And you reply, "Oh, I'm battling infertility so I am wearing these stones to increase my chances." I think infertility is private for a lot of people. So if you're relying on word of mouth to drive traffic to your website, or if you are hoping that women will think to google "jewelry infertility" and hopefully find you (if you have good search engine optimization, which costs money...), you may have trouble getting enough hits to make the fees worth it.
I also think a lot of women dealing with infertility are already spending a fortune in time and money on doctors, hormone injections, counseling, and charting their cycles etc. - not sure how many would add on the expense of jewelry and the time needed to find it.
It also takes time on eBay to build up a reputation and become a preferred seller (I think that's the term - I could be wrong), so you have to absolutely have the long view here and not be expecting a lot for the first few years. And yes, the fees are hefty.