We do that all the time. The first order is just that, and is an indicator that at least someone wants the product.
If a customer pays for units only, there is no conflict. If they pay any one-time fee, we make it clear in advance whether they have any design rights. We prefer to make it clear that we are customizing our proprietary technology to their needs and that they have no ownership. Somtimes they have exclusivity for a limited time.
With no NDA, the seller can sell to others, but has no IP protection beyond the usual copyright or patent.