Sales people are the eyes and ears of any successful product organization. They are the ones who interact with customers on a daily basis and who are the best judges if product-market fit was found. Unfortunately, they tend to follow the path of least resistance and always follow the immediate buck (i.e. they skate to where the puck is, not where it's going), which might be profitable in the short run, but often leads in dead-end situations. Therefore their feedback should be critically judged and should be just one of the many input signals for defining a product roadmap.
The most effective way to garner sales feedback is to listen to as many sales calls as possible. But since the sales people do lots of sales call, listening to all of them might be too time consuming on the product manager, so asking sales people to write down extensive meeting notes is often an effective strategy and something they already do as a part of normal sales routine. A product manager should watch, though, that selection of prospective customers is not entirely left for sales people to decide. Namely, the sales people have selling hypotheses of their own and they tend to carefully select who they call. This might lead to a skew view of the market, so product manager must have alternative ways to interact with the market and some power to influence sales which leads to chase.
This is part of series of posts on managing sales.