"Mint" of course has a different meaning for Leica buyers. Whereas with most brands, "mint" in the sense of "totally indistinguishable from new" would probably go for 25% more than "Exc [Excellent] ++" (which would be "very slight signs of use"), Leica fanatics will pay double (but ONLY if the "mint" objects genuinely have NO signs of use and are preferably in their original packaging with all tags, documentation, etc.).
I am not a collector, usually if I buy second-hand gear I would go for between mint and Exc++, with Leica I would not dream of buying mint because it is so much more expensive. Ballpark figures for the UK would be £500 - 600 for Exc++ (very clean user) M4 and M5, twice as much for genuinely mint, maybe even three times as much for totally mint in box with all packaging. M6 I would say £600 to £700 for Exc++, maybe 50% for mint, as the M6 is a newer camera and there are more around in mint condition.
I might have a doubt as to whether you can get full price for collector-grade Leicas on e-bay, where buyers cannot view the actual goods before purchase. If you do list any goods on e-bay as mint, I would be sure to include lots of clear pictures showing them from all sides.
Regards,
David
PS: The prices I have mentioned are dealer prices including VAT (sales tax) of 17.5% and guarantee.