Serial single founder here. Start building and start selling. If you can’t deliver a product to a paying customer solo then you’re not cut out to be a sole founder.
I agree with this except switch the order. First, start selling. Then start building. It is so easy to waste so much time building something nobody wants, and you won't know until it's too late if you try to sell it afterward. Selling first will inform your decisions on what to build.
If you are the sort of person who enjoys the making side of things, and aren't naturally gifted at selling, this can be a trap. I've seen so many solo founders try to validate a product by selling before building, only to get talked into different features and use cases until they're stuck building some bloated mess of a service. Not super enjoyable.
That said, if selling is your strong suit and you have the discipline to sell a product idea without deviating from your product idea to chase sales, then selling as a validation tool is fantastic.