Some products are hard to build, but easy to sell. Early-stage startups like this, with a lot of technical risk, should focus on building credibility thru prototypes w/ early adopters. Some products are easy to build, but have little evidence of market demand. These startups should focus on validating demand w/ sales.
More notes on how this concept influences strategy for early-stage startups and how to tell which bucket you fall into: https://fastertim.es/post/validating-startup-ideas/