Can a buyer back out of an NDA just because they never got a real offer?

No — a confidentiality agreement's obligations generally don't depend on whether a deal actually happens. An NDA is typically drafted to survive regardless of the outcome of negotiations, for a defined period stated in the agreement itself, precisely because the risk it protects against (misuse or disclosure of what was learned) exists whether or not the buyer ends up making an offer, receiving one, or completing a purchase.

A buyer who walks away from talks because they never got the terms they wanted hasn't been released from what they already agreed to about the information they received — those are two separate things. If a buyer tries to argue that no deal means no ongoing obligation, that argument generally doesn't hold up against a properly drafted NDA's own wording, which is usually explicit that the confidentiality and non-use commitments continue independent of whether a transaction results. If you're concerned a buyer might take this position, or already has, review the specific duration and survival language in the agreement you signed. A Treadstone business lawyer can confirm what your agreement actually says and how to respond.