Why wouldn't a potential domain purchaser be a little suspicious of someone selling a domain name they claim generates a lot of traffic?

It's not wrong for being a little suspicious when you buy something online most especially if you don't know the seller. This is a very normal reaction and this is very common in online auction.

Its worth being suspicious, but there are a lot of reasons they may want to sell it. Perhaps they need a few months worth of income NOW instead of a small amount each month. Perhaps they tend to cultivate sites, move on, and develop something else every now and then.

Or, perhaps they've inflated the figures and are scamming you.

The best thing to do is check the page rank, alexa stats, and other similar tools, check how many backlinks they have, and ask to see the stats they are using - preferably a publicly accessible tracker, rather than screen shots, as they can be doctored.

I agree with etali. Be careful, but there are legitimate healthy businesses sold in the bricks and mortar world for a variety of reasons, too. Although these may be easier for a potential buyer to evaluate.