As a hobbyist or a collector - Once the machine leaves your possession, then all bets are off. The buyer is on his own. Exactly the same as a car.

As a Business - I think it is good business practice to offer or state a warranty on the machine for between 1 month to 3 months. As a hobbyist, collector and business, I always offered support over the phone if any issues occurred. I've never had any major issues except related to transport. Once it leaves my place, I only "hope" the games don't get mistreated or handled badly.

Dealers / sellers can minimize warranty issues if the machines are reconditioned properly. But the games are usually over 20 years old, so this must be kept in mind.