If you had to ask such a question, it means you are worried about consequences. That automatically in my book means that buying the theme, making sure all works, and then selling the site is the best bet for you.
Gaming search engines is not illegal and neither is testing out products prior to purchasing them. Yet when you accept cash for property you are testing out, they are gonna... do nothing.
I myself would charge the buyer a premium for getting a limited edition theme, possibly nulled by an 8 year old Russian prodigy. That's gotta be worth a few extra quid. Tell the buyer not to worry, with all the back-doors, you will personally ensure that his website is protected and all customer info is protected against rival hackers. In fact, inform the buyer once they get comfortable about the mandatory monthly insurance payments they need to start making.
If the developer complains, remind them of all the free publicity they received all thanks to your hard work posting their nulled product on download directories; and being a well mannered gentleman that you are, you never even asked them for anything in return.
Since now you feel insulted by their complaints of you selling their nulled theme, perhaps the developer owes you 30% of the net revenues as compensation, back-pay one might say, along with all the pain and suffering their insinuations have caused.
If this is not how you see this situation, just pay for the theme and focus on things within your comfort zone. SEO is stressful enough.
