Using Wordpress to Sell Leads

I generate leads on my Wordpress site. Here's how it works:

1) A member of the public fills out a Gravity form on my website, requesting information about a service

2) The Gravity form creates a custom post type (called 'Leads')

3) The professionals, who are members of my site, can then purchase access to that Lead (i.e., pay to see it)

4) The Lead is limited to only one sale, so the professionals can purchase each lead on a first-come-first-serve basis

I know how to do steps 1 and 2. I am looking for advice on how to do steps 3 and 4. I have found both membership and paywall plugins that allow me to sell access to individual posts. The trick seems to be limiting each post to only one sale (after all, each lead should be sold exclusively to the first buyer).

I don't need to go through a third party lead buyers network, as all the buyers are already members of my site.

I am trying to enable these buyers to buy leads without a lot of manual work for each lead on my part.

Does anybody have any suggestions for how they handle lead sales through a Wordpress website?

Thank you.