These are just a few ideas. To find out what other people are collecting, and what's valued highly, I like to follow the auctions at eBay , and other online auctions. You can see what other people are asking for their collectibles, and get a good idea of what's currently valuable and what's not. You'll need to know the current market value. There may be a fee for this service, since the dealer studies your item and gives you a written report on what it is and what it's worth.
Fees vary according to the amount of work involved in the appraisal.
You could simply present the item for sale and ask for an offer, but don't accept the first offer you receive. Get a few competitive offers. Try to sell to a specialist - sell dolls to a doll specialist, etc. Be sure to check out the online auction at eBay . If you sell to a dealer, the dealer's margin of profit will always be figured into the price.