A buying signal may come as any of a variety of types of often subtle cues that the mood has shifted, and the other person is now ready to agree . . . or at least to be open to what you propose.
In other words, a "buying signal" may signal readiness to actually purchase, but may also signal a readiness to take other action toward ultimately buying.
You may also encounter gatekeeper buying signals that indicate readiness to take interim steps along the way---such as to hear you out, to ask for more detail, to ask for proof of what you claim, or, perhaps, to attend a demonstration of your product in action.
Buying signals from gatekeepers or screens may come as words, as facial expressions, even as body movements such as relaxing tension, leaning toward you or your product to see it better, and so forth.