Did you know that MTD go threatened with EPA fines for selling the tools to their dealers without requiring them to also have the special equipment to insure that the adjusted carb still met emissions standards. Also dealers where also threatened with fines up to $41,500 per tool for selling them, and that they could also fine the person that you sold it to up to $4,150. That was the reason that the manufacturers stops selling the tools to their dealers.
Here is also one that most people don't know. A dealer can be fined for installing an engine on a piece of equipment that is older than the engine that was removed. So you cannot legally swap an engine from an older mower to a newer mower. Also you cannot legally adjust the carb on a piece of equipment that you are not an authorized dealer for. So even though your MTD tool will also work on an Echo doesn't mean you can legally use that tool.
I know of one Toro dealer was raided by the EPA because their distributor had released his spring stocking order too early, and that order contained EPA certification stickers for the following year. So he basically received 2014 EPA stickers in December of 2013. For the most part is hasn't been enforced but the potential is there for some real headaches.