You can also do a search of competed eBay auctions for similar dolls to see what people have paid in the past. Read the descriptions of the dolls carefully. Details about the condition can help you decide how much to bid. If you have a question or are unclear about something in the doll description, you may want to e-mail the seller for more information before placing your bid, especially for more expensive dolls. Ask specific questions.
Most sellers are happy to answer.
In particular, you may want to ask what the doll's flaws are so that the burden of disclosure is on the seller, and you are not unpleasantly surprised when you receive the doll. You can set your bid for the maximum you are willing to spend. If someone bids against you, eBay automatically raises your bid so that you remain the high bidder until the bid amount reaches your limit. If there is a lot of feedback, at least glance at the recent responses and any negative feedback to see if there are any comments of concern or issues you would want to clarify with the seller. For example, you may still want to buy from a seller who received a comment about not packing breakables well, but you may want to clarify how they will package your item.