Hello Judi, welcome to Stamp Community!

FDC's value depend on the artist's name or artist company name on the FDC or the artwork being recognized in a FDC catalogue.
The better art is of course worth more to more people or potential customers.
Very nice art on a hand painted or hand drawn FDC can be worth a lot.
Topical ideas on cover like popular collecting areas such as birds, cats, dogs, flags, famous persons, ships, and other popular subjects can be worth more to their respective collectors.
A common stamp done up right with nice artwork on a FDC can be worth more than just one cancelled without artwork.
ebay can be a good place to sell, but as there are more than 100,000 listings on
ebay right now with the term FDC in the title, the better chance is of selling them cheap or having something special about them that makes them worth more.
A dealer will give you a small percentage of what a collector will because he has to sell them on himslf. A collector will pay the highest.
Can you tell what you have or show pictures or scans of a few?