I don't think there is any problem posting an ad for used equipment. Most forum would encourage this type of posts, compared to the normal spam where companies specializing in selling equipment, services, ...

The biggest problem may be the interface - there are lots of virtual machines allowing use of DOS on a high-end PC with the latest and greatest operating systems but such virtual machines often ends up short if the DOS environment needs to access special hardware. Another issue is that a lot of programs from the DOS time doesn't take into account that a computer can be 1000+ times faster than when the program was developed, which may seriously limit the availability of machines that may be used.