As a threshold matter, let me be clear: not all agreements are enforceable under New York law. And the reasons for this vary greatly: some agreements will fail simply because they weren't reduced to writing (that's not to say that all agreements must be written; some can be proven by other means. For more on this topic, I suggest you download the Free book for New York consumers, "When You Don't Have a Written Agreement"). Other agreements may be held invalid as a matter of public policy because they inherently call for illegal conduct.
But assuming that your agreement doesn't fall into one of these categories, a plaintiff is, generally speaking, required to demonstrate the following in order to win a breach of contract claim in New York:
(1) that there was an enforceable agreement;
(2) that the plaintiff fulfilled his obligations under the agreement;
(3) that the defendant breached the agreement; and,
(4) that the plaintiff sustained damages as a direct result of the defendant's failure to fulfill his obligations under the contract.
Of these 4 criteria, the one that dooms more breach of contract claims than any other - particularly where there is no written agreement - is the first: proving that the two sides actually entered into an enforceable agreement. And here's why: unless the plaintiff can show that this specific plaintiff entered into an agreement with this specific defendant, the breach of contract claim will fail.
This issue has come up repeatedly in the breach of implied warranty arena, where the courts have dismissed these claims that were made against affiliates or upline distributors that had no direct relationship with the plaintiff-consumer. In legalese, this doctrine is called "privity." See, e.g., Chen v. Street Beat Sportswear, Inc., 364 F. Supp. 2d 269, 294-95 (E.D.N.Y. 2005).