Many companies do this type of cross-assignment as a way to build bonds between their sales and marketing groups. However, even if you're not planning such an assignment, you should still ask this question because it takes the theoretical situation in the first question into the practical realm of the candidate's career.
The worst possible answer is something like, "I'm sure I'd be good at selling, but such an assignment would not be the best use of my talents." A response like this suggests that the candidate, if hired, will have difficulty creating marketing programs and deliverables that are useful to the sales team.
This is a huge problem, according to the book The Profit Maximization Paradox, which quotes surveys showing that salespeople spend a whopping 40 percent of their time preparing "customer-facing deliverables" while using less than 50 percent of marketing-created sales materials.