You will need to have your painting appraised by someone with some credentials, so it's good to check with your insurer before contacting anyone to learn who they will accept.
If Carpenter sold his work by himself, and never used a gallery or another person, the family may be the only 'expert' in your painting. Your insurer may want to see some copies of his sales receipts or other records.
Then, if you know Mr. Carpenter's agent, gallerist or family, I would ask them. They could give you good provenance for your painting and good financial info to provide to your insurance company.
'Provenance' is a record of how you obtained the painting, from whom, and when it was given. If you swapped your labor and gave Mr. Carpenter an invoice for what your charges were, that itself may establish the painting's worth in part. If his paintings are more valuable than your charges, that's something that the insurers need to know. If money changed hands, then what you paid also is valuable info.
If Carpenter's works are popular and his market is strong, you should add some additional insurance. Art is a commodity, so if you keep the painting with no intentions of selling it, be aware it may not become automatically more valuable as the years go by. Once a value is established, it's always good to check on how his works are doing in the market from time to time.
eBay is a crappy way to try to establish value. Too much of eBay swings too far, and essentially, every bidder is a bargain basement hunter first, and very often gets caught up in bidding fever later. You can go nuts trying to establish a base value from looking there. Don't presume anything if you have no clue as to how his sale prices were when he was alive. You may be surprised or disappointed if you go by your guess, and your insurance company probably won't take an eBay record seriously.
Here is a much better net source, but it will cost you $13.50 for a 24 hour subscription.
http://www.askart.com/askart/c/georg...carpenter.aspx
This should give you all you need to know about how his work is doing now. $13.50 may be worth it, depending on how much trouble it is for you to dig the info up from the methods above.