In most cases (private parking lots) it isn't illegal to take a disabled parking space. The fines a private company levies against you don't have any merit (though of course they can refuse you entry and sue you for damages or whatever).

Parking spaces on public roads is a separate issue, and it's more in line with driving in a bus-only lane (technically illegal but it is because you need a particular permit -- so it requires more than just being disabled).

My point is that your are jumping the gun with discussions of legality here. The litmus test should be wheter it's shitty to lie about a disability in order to cut through reselling bullshit. Other commentors have more than adequately covered this topic though.