There are plenty of ways to present yourself that won't seem desperate, such as saying that you feel that your original vision could be better realized as part of a larger organization (either because more resources or complementary offerings) or the perfectly acceptable "founder wants to move on to other things"

Any buyer will be more interested in the potential post-close than in why you sell. Appease their fears about the quality of the business and the transition (retention of talent and management is often the #1 risk when buying a small business)