I've had this exact same question. I've asked local reefers and the seem so offended but then I remind them that I also spent lots of money and time keeping a clean system. So the answers in my opinion is "YES" you should disclose to the buyer it's good customer service, you should also be prepared to respond with what you're doing to address the issues and how they can also guard against pests. I would be ticked if I got aptasia, red bugs, flatworms, majano, vermited or any of the pests in my system, which I have gotten from a LFS and a local reefer (I will never buy of get anything from them again). Just think of it this way, if you are selling pieces that have pests where do you think your reputation in the reefing community will stand especially as a small seller? You probably would never sell anything again...
I don't think it's rude to ask... you should know the reef keeping practices of those you buy from.