[adrianthedev]

As a solo developer I get asked a lot on sales calls "How many paying customers do you have?"/"Are there paying customers?" and other similar questions to evaluate the size of my business.

I guess it's fine. It's natural to know the partner you're about to take on has the incentive to continue developing their business and still have an "alive" product in the future.

[marcinzm]

Those are the same questions we ask any smaller company we are thinking of integrating with. Depending on the cost of migrating off we may even ask to speak with some of their other clients.