There is plenty of old hardware that won't properly work for modern linux distros, either by missing firmware packages or software not running on the CPU anymore (such as on an old Thinkpad of mine from 1998, modern distro's don't properly run, a lot of stuff fails with invalid opcodes or segfaults. An ancient copy of knoppix runs (with kernel 2.3))

Even then, most people don't buy with "it must run linux" in mind, they grab whatever has good sounding numbers on it.

Heh, wasn't thinking of going that old, more like a Thinkpad T430 (can be had for $150) or newer depending on budget. Any of that line will run your distro of choice without issue.