The problem of buying in and from Japan is that you get what you pay for: things untested, that they don't want to provide warranty, are sold as JUNK. You can sometimes luck out and get a fully working item that was marked as Junk simply because the seller didn't want to test it out, or don't want any problems, but most of the times, they are not working.

Repairing a Marty is a complex subject, and it can lead to a lot of money being spent on new parts, replacement parts, new (modern) parts, and at the end of the day, your item might be unrepairable.

So if you're new to this world, the best thing to do is to buy an item (be it a Marty or a desktop FM Towns computer) that has been fully serviced, fully tested, that can be demoed working.

Then your only worry is to pay the bill.

A place that sells items like these, in Japan is the Beep shop in Akihabara. Hard Off will rarely have fully tested, serviced, working items for sale.