It just occured to me. Why would Mrs. Dashwood need to sell older jewels? I don't recall anything specific being said about the reason, but I have some speculations.
Are the Dashwood ladies living beyond their means, despite Elinor's attempts to have them live moderately?
Or, are they to pay for the improvements that Mrs. Dashwood wants for the cottage?
I assume that the jeweler could resuse not only the stones, but if the settings are in gold or silver, he could melt that down and reuse it.