The liability that most don't think of or pay attention to is the EPA regs on the carbs. From what I understand most dealers are required to keep limiters or reinstall limiters when they are missing. I agree that most saws come from factory to lean to make it to a decent amount of usable hours. Seems like there is some ridiculous fine for removing limiters but how many of you pay attention to that? At the same time its comical how many AM manufactures sell carbs on ebay that don't come with the limiters...
I would venture a guess that 99.9% of saws wouldn't meet EPA standards after they have had muffler mods, carb adjustments, and/or port work. Then again EPA is only paying attention to new units being sold in the USA. Does anyone know someone that actually got fined for modding their saw (beyond getting clipped for not having a spark arrestor in)??
Hind-sight is that we are a very small group in a big pool of chainsaw operators/owners. ALSO we are talking USA market here and not UK/EU/AU/etc...
Legally I think there is always some liability when selling used equipment if something should break and cause an injury.
As for saw mods being "main stream"... I think saw mods are done on an overall very small percentage of saws for guys that know what they are buying. Most saws are bought and used in stock condition by owners that know nothing about "mods."