welcome aboard.
from the tone of the thread, I assume your husband is no longer with us, for that I am sorry.
this is the time to sell it as the boating market is a sellers market.
you mention that the boat has been sitting. sitting is bad for boats. so is being left outside.
list the boat as a project boat, that has been sitting. state that it has been sitting outside. unless you know the condition of the motor, drive and the boat structure (fiberglass boats use wood as a structure, and generally that wood rots within 15 years), it should be considered a project on a trailer
as a project boat goes, its probably worth $500 to the right individual.
if the motor turns over and has good compression, it may be worth $800-$1000
places to list the boat are your local classifieds, facebook marketplace, ebay, and craigslist