There's a giant tank/armor auction every few years at Aberdeen Proving grounds and at Camp Grayling. Last one was in 2015. Usually it is ones from WW2, Korea, Vietnam era but there are a few from the late 80s/early 90s entering the market now. All perfectly legal provided you go thru the background checks, the mile high stack of paperwork and the 90 day waiting period to become a registered buyer. It's not cheap though, you're looking at half a million or better for most old tanks, newer abrams and leopards, you're looking at a million plus. Then there's the actually tax stamps and paperwork fees.
heck, you can actually buy used US fighters. There's a guy in Indianapolis that owns an F4.