What is moral and what is legally moral are two mutually exclusive concepts. As a lawyer (and this is NOT legal advice) it would appear that there is a valid and binding contract here. The laws of your state may differ slightly, but the essential elements of an enforceable contract are generally known to be:
You made an offer (step 1) that buyer accepted without materially changing the terms of the offer (step 2); there was a mutuality of obligation or meeting of the minds between the parties (step 3) in agreeing to the essential terms of the contract, i.e., the parties agree to the same thing in the same sense at the same time; there is certainty of the subject matter (step 4) that would enable a court to understand the parties' obligations, i.e., machine in exchange for money; consideration was paid by the buyer (step 5); and the parties are competant to enter into the contract (step 6), i.e., no one is a minor or mentally incompetant.
As RobT notes, his damages are probably nil, but he could sue you for specific performance of the contract -- to make you deliver on your promise to sell the machine. If buyer passed up an opportunity to buy another machine because he was going to buy yours or drove to your house (expending time and gas) he very well could be "damaged."
Even if there wasn't a binding contract here, buyer may be able to sue you under the theory of promissory estoppel whereby you made a promise to sell, that promise was reasonably relied upon resulting in legal detriment to the buyer and justice requires enforcement of the promise. So even where there isn't a valid contract, a court may, in equity, create a "contract" based on this theory in order to achieve an equitable result.
So... whether there is a legal contract or not, buyer may be able to get the machine from you regardless of what you do at this point.