Can a buyer who signed an NDA still use what they learned to compete with me later?

It depends heavily on exactly what the NDA says, and this is one of the most important drafting details to get right. An NDA that only prohibits disclosure — telling others what they learned — may not stop the buyer from using that knowledge themselves once the deal doesn't happen. A separate, explicit non-use clause is what actually restricts a buyer from applying insights about your pricing, customers, or operations to compete with you, and it needs to be there in clear terms, not assumed as implied.

Even a strong non-use clause has real limits. It generally can't stop someone from using general business or industry knowledge they already had, or information that becomes public or independently known later, and courts are typically cautious about enforcing restrictions that are broader than necessary to protect legitimate confidential information. If you're negotiating with a buyer who is, or could become, a competitor, this is exactly the kind of case where the confidentiality agreement deserves careful, specific drafting rather than a generic template. A Treadstone business lawyer can make sure the non-use language actually covers what you're worried about.