First I would do a search on the buy-sell forum at
Fred Miranda Forums. Use the search function and narrow it down. They do a large enough volume of Leica buy and sells that you should get a fair idea. Generally items sell for listed or just a few percent lower than asked.
A second resource would be to find out what
KEH or
MPB will offer you on a straight out sale. They will offer about 66% of what they expect they can sell it for.
An additional resource would be to ask a Leica Store (not to be confused with a store that sells Leica...) They will give you an honest answer and may offer to buy it from you for resale. They too might offer between 66%-80% of what they expect they could resell it. Leica Store Miami has an exceptional reputation.
EBay prices seem to be the hardest to use as benchmarks, because, well...it's Ebay.
All that being said, do not expect that you could easily command the same price as KEH, MPB or a Leica Store can for used equipment. They offer buyers limited guarantees and their reputation is on the line and buyers are willing to pay a little more for that assurance from an established business.
On a private sale, anything you can get over the KEH/MPB/Leica Store offer is the premium you receive for dealing with the hassle of a private sale.