the ones i'm finding in ebay say that it is not recommended for 64bit windows.
That's because they are pirated or cracked copies.
If you buy a legitimate Mach3 license, Mach3 runs fine in 64 bit operating systems, provided you use a motion controller like the UC100. But if you have to buy a UC100, UCCNC is $125 cheaper than Mach3, and it's better in most ways.