Booked a Delta flight where husband and I are not sitting together. I ended up with a preferred seat 26A on a MD-90 plane. No seat in front of me- by window. I would rather sit with husband. Could I resell this seat once on board to someone closer to my husband. Or is this in bad taste?
This would, indeed, be in bad taste. I'm not sure there's a specific rule for the situation, but airlines generally frown upon soliciting their passengers, and they view seats as something they assign and do not guarantee, not property to be sold.
It is, however, not uncommon to offer to switch seats with someone in order to allow families to sit together, without any money changing hands. You could approach someone sitting by your husband and offer him your seat, explaining that it comes with extra legroom. He may take you up on the offer, but might have particular reasons to prefer his current seat (it might be next to his family).
There is an app called Seateroo that's intended to create an online marketplace for seat selling. I've never used it, find it pretty ridiculous, and would imagine that the odds of finding someone else who is sitting near your husband and wants to buy your seat on your particular flight is pretty darn low, but it does exist.