If a guy sends you an engagement ring, and you sell it, can he sue you for the cost of the ring?

As written, it almost, or perhaps more than almost, appears the poster is planning, in advance, to ditch the engagement ring upon receipt but seeks to cover the bases regarding personal liability for the action. If that appearance is accurate, it seems to at least potentially embody larceny by trick. This could of course create criminal law consequences for the person seeking an item under false pretenses, in addition to any potential civil law consequences.

Did you say yes? How long did you have the ring before you sold it?

An engagement ring is a conditional gift which has to be returned if the marriage does not go through. Yes you can be sued.

Acceptance of an engagement ring is an implied promise to marry, so unless he's the one who broke off the resulting engagement, he probably does have a viable claim.