As you can see from the wide range of replies, there is no "standard" here.
Your concerns should be both ethical and legal. Your jurisdiction may have laws that govern verbal contract law, define what a deposit is, and whether the default is refundable or not. Before you decide to keep the deposit, it behooves you to do the necessary research to understand what the law is in your jurisdiction, so that you can make sure you don't violate the law.
On the ethical point, it seems to me that the correct approach when taking a deposit is to make 100% sure both parties are clear and in agreement about the nature of the deposit. Lots of situations involve refundable deposits, and lots involve non-refundable deposits. It's very easy to have a misunderstanding, and a written agreement ensures both parties know going into the deal what they are getting and what their obligations are.
It's obvious you failed to take that step before accepting the deposit. As the owner of the machine, and the person who is holding onto the cash, you also have the upper hand in the deal, which IMHO gives you an ethical obligation to give the other person the benefit of the doubt. You've said yourself you were in no hurry to sell the machine, and it's doubtful that you incurred any real financial loss by the delay in having to relist the machine, so it's not like the $200 is compensating you for anything real.
In this situation, if I were you, I would return the deposit. And next time, make sure you don't engage in ambiguous verbal contracts. It is trivial to put in writing what you're agreeing to. Just do it.