Does relying on a single patent or trademark make buyers pay less for my business?

It depends heavily on how strong and well-protected that single asset actually is, more than the fact that there's only one of it. A single patent or trademark that is properly registered, actively enforced, and central to a clear competitive advantage can support a strong price precisely because it's hard for competitors to replicate. The same single asset, if it's poorly documented, close to expiring, has never been enforced against infringers, or is easily worked around, gives a buyer much less comfort, regardless of how much the business currently earns from it.

The nuance buyers focus on during due diligence is less "how many" intellectual property assets you have and more whether the ones you do have are properly registered and actually enforceable, and whether they clearly belong to the business rather than to you personally or to an arrangement that was never formalized. A valuable-sounding trademark that was never properly registered, for instance, offers far less protection than an owner might assume.

Before going to market, it's worth confirming your registrations are current and actually held by the correct entity. A business lawyer can review your intellectual property position and flag gaps that would otherwise surface, at a worse time, during a buyer's due diligence.