I think people should have to validate their xenforo licence before being able to download xenforo add-ons from the marketplace.
Why...? First: the buyer might not be the owner of the license for where it's supposed to be installed. Second: why do DP care whether or not the buyer has a valid license? Third: why do the seller care that the buyer has a valid license?
If I was selling a xenforo add-on, I wouldn't want it to be used on a pirated copy of XenForo. The buyer could get a validation token from the owner of the site. DP should care because they shouldn't want to make marketplace add-ons readily available to pirates.
And again... Why? What do you care whether or not it's being used on a pirated version? You've gotten paid. How would this "validation token" work? What does it say? What is it checked against? Does this already exist, or is it something that would have to be developed? And again, why would DP care? Their responsibility goes to (at best) make sure that the add-on is legal to sell.
Yes, I checked, and it seems this is already in place - so why bother with having it here on DP? If the developer cares, then s/he can build it into his/her plugin/addon, right? Then do that.