The eco-boons of renting/borrowing tools instead of buying them aren't exactly blatant since it's difficult to really pinpoint the environmental woes associated with buying new tools. As far as I know, there are no studies on how much CO2 the manufacture of a new table saw or buff sander produces. Essentially, you're avoiding the resource-intensive manufacture and packaging of a brand new product. And you're sharing something rather than keeping something for yourself ... that's certainly a Mother Nature-approved action if there ever was one. Plus, given that renting/borrowing is the more frugal choice, you can redirect the money you're saving from not buying new tools toward other green home improvement projects.