Checking the sold listings on ebay is usually the best way. Sold listings should be the priority over just completed(but not sold). Sellers can set any price they want, but if nobody buys it, it’s not worth the asking price. Also, try to find as many as you can. Sometimes circumstances(crazy new buyer, exceptional condition, weird bidding war, etc) that create a fluke auction with an astronomical price. If you can’t find any similar items, you can always ask around in the pony community.
If you’re selling on ebay, you can always just put it up as an auction, not a BIN, and let the market decide what it’s worth. Also, if you want the maximum price for your items, spend some time making the photos look good. Wash and style the hair, light it well, and learn how to take really good pics. And give a detailed description - especially the flaws. Nobody liked a nasty surprise, and taking the time to describe any problems 1) self-selects out any buyers who don’t want those flaws, and would return the pony anyway, and 2) gives all prospective buyers the confidence that you examined the pony carefully, and are giving an honest description of it. Spending the time to present your ponies well can make a world of difference in the ending price, and it can mean the difference between an item that sells quickly, and one that sits there for months.