Can owner sell the apartment to someone else after I have already signed the contract?

It is difficult to prove that an agreement is final until signed by both parties. A helpful fact pattern is if an agreement is marked up by the counterparty, sent to you, which you then sign without comment. This could be argued as offered and accepted, although in practice it would have to be litigated to be resolved.

If countersigned, the agreement is binding as executed.

Why would he hold an open house if you've agreed to his terms? I'd give him an exploding offer. "This offer expires at midnight on Friday." You're letting him take advantage of you otherwise.

Thanks for your comments and suggestion. I am getting about 6.5% off his asking price. I think he is hoping that someone might be willing to pay him more.

They are trying to use your offer to get someone to bid higher. They will use this at the open house. They can't force you to bid more. They can just try to play you and any other potential bidders off of each other.

I would be prepared to walk away from any apartment. There are plenty more.

If I were you, I would send a letter to broker rescinding your offer. It is really a sleazy thing to do (not unexpected in NY of course) and you should make them sweat that you may walk away and I think you have some strong standing legally since they are continuing to market the property that the offer has not been accepted.

they should also be countersigning within the week

"I think you have some strong standing legally since they are continuing to market the property"

There's no legal standing question here. From a legal standpoint, what you have given him by delivering a signed contract is an offer. Nothing more, nothing less. He may accept the offer only by signing it. At that point, it is accepted and legally binding. Prior to that moment, you may revoke the offer at any time, unless he has given you consideration in return for holding your offer open for a specified period (this is "consideration" from a legal, not financial perspective). If he marks up the agreement and signs it, your offer expires, his markup constitutes a counteroffer (a new offer, legally speaking), and he forfeits the right to accept your offer.

What people fail to grasp is that the real estate "offered and accepted" is not equivalent to the legal definition above, because it lacks specificity. What has happened to date is that you said "I'll give you $1.1 million" and he said "OK, I'd take 1.1" That is not an offer and acceptance because it is not specific enough. You'll give him $1.1 when? In what form? Subject to which representations and warranties? Subject to what termination rights?

This is why he has legal standing to show the apartment. He has not accepted your "offer". He actually has your offer in his hands and is deciding whether to accept or not. You may withdraw that offer without consequence. By the way, if his lawyer cashes your check, the offer is accepted regardless of whether he signs.

I understand this reality now. But just feel it’s not fair to me. The seller can hold my signed contract and check for as long as he wants to play the bidding war!

Every offer to purchase I've dealt with in the last 30 years, both as buyer and as seller, had a date and time by which the owner had to accept the offer. If not signed by that time (usually 24-48 hours), the buyer's offer is void and the earnest money is returned. Did this go the seller directly from the agent without your lawyer seeing it? That's not unusual, but these days agents usually ask me how long I want to give the owner to respond and write a day/time limitation into the initial offer.

I had one like this a few years ago - I was buying in Florida, it was an estate sale and the owner (a lawyer herself) was in Seattle and I was in New York. I signed on Wednesday and wanted the limit of acceptance to be Friday, but the agent insisted on the following Tuesday. I knew right away what was up and found the open house ad for the property in the online Palm Beach Post. The owner signed on Tuesday.