Really depends on the AIO, but chances are good you're looking at an HDD at best, as Gam3r01 stated - although you could potentially reuse your OS, if the AIO is heading for the trash otherwise.
Some AIO's did use desktop CPUs and RAM, but they weren't overly common - typically soldered Pentiums were used, which isn't worth salvaging.
You'd likely be better off selling the AIO complete - with the HDD and OS included, and using that cash towards your build.