I've lived situation. Large co bought one of our systems, as their internally built systems weren't up to the task. We sold it at effectively break even. This was in part due to my business partner's view that we could turn this into a bigger deal.

Well, no. We couldn't.

The "customer" wanted specifically to see how we did what we did, in order to copy this. It took me a while to figure it out, but I did, and we wound up walking away from it. Partner was angry, but it was absolutely the right business decision to terminate the interaction.

PSA for any small startup firm with real differentiation: BigCo will, absolutely, positively will, try to see if they can replicate your value on their own rather than buy from you. Patents won't stop this. Pretty much nothing will, apart from an architecture and implementation that they don't understand. They will mess with you. Approach with extreme caution, and be prepared to cut bait quickly.