Dallas doesn't use Lewis much. In 2018 he was on the field for 18% of defensive and 20% of special teams snaps, adding 8 plays on offense, to make a total snap count of 279. The vast majority of these snaps came in weeks 5, 6, and 16. He recorded 156 snaps in those 3 games alone.
So, regardless of how we feel about Lewis, the fact of the matter is that Richards doesn't use him. When you've got a talented player that doesn't play, then they are either depth or should at least be shopped around.
As for depth, last year we were lucky in the secondary and didn't see many missed games. We really dont have a lot of depth right now and most of the depth is Lewis. He is "the guy" after our big 3 (Jones, Awuzie, Brown), as he is 4th in snaps at CB. He's 4th by a mile but behind him the next guy has a total of 53, only 3 on defense. This explains his sporadic usage with most snaps coming in 3 games.
So,
he's a very important depth piece around here.
Now consider marketability. We all feel he's a good player with potential. I would argue he's got way too much talent to be a distant 4th on the depth chart. I wont go into detail here because it seems a lot of us agree. One minor strike against his value is the lack of 2018 game film.
Overall,
he's highly marketable.
Finally, I want Richards to have guys that match what he wants to do. I'm not going to pull a JJ and keep a guy around if the coach I've entrusted with the team (or part of it anyway) doesn't want to use him. I'd much rather try to bring in a guy that Richards can and will use. It only makes the defense better to have 4 CBs the coach likes instead of 3.
Altogether, I like the OP's idea of trying to trade Lewis. I think his specific trade is dumb as rocks for all parties. You have to leverage his marketability in a way that allows you to replace the depth he provides.
I would do a player-for-player trade if another team wanted a Lewis but instead had a guy Richards likes, assuming the financials make sense.
I would also consider signing someone that Richards likes to be the CB4, trading Lewis for a midround pick, and then drafting a CB Richards likes. CB4s are going to be cheap.
Lewis is too good to have sitting around doing nothing and if we can get something for him without sacrificing that CB4 safety blanket, then we should.
https://www.pro-football-reference.com/teams/dal/2018-snap-counts.htm
https://www.pro-football-reference.com/players/L/LewiJo01/fantasy/2018
