Brighton fan here. Dan Burn has just gotten better and better for us. He has played left of a centre-back trio, left wing back and left of a back two consistently well for us. That said, he is viewed as a back up to Dunk and Webster mostly, who are two superb centre halves so no shame there.
He isn't the quickest, but uses his positioning superbly well and always gives himself a few yards. In the wrong environment against lightning fast players, he can be made to look very slow (Traore and a few others have given him nightmares at LWB) but this is quite uncommon.
He is surprisingly bad in the air. Whilst he wins defensive headers, his percentages aren't as good as they should be and the ball can end up going practically anywhere. Attacking corners is the same, often wins it and puts it into row Z.
He has improved tenfold since he joined and Potter really rates him. After a poor performance last season he was getting some stick, Potter said afterwards "anyone who doesn't think Dan Burn is a good defender doesn't know anything about football".
As for the sale, I can see it happening, although I have no insider information, I am just a fan. His contract is up in June 2023.
The issue is the fee. Tony Bloom is, in my biased view, the shrewdest operator in the league and as a club - despite large losses - we have no need to sell unless a player forces a move. As shown with Ben White (we rejected £25m then £30m from Leeds before he'd even kicked a ball in the PL), he simply won't sell unless he gets his figure.
Bloom sets a figure and just outright declines every offer until that figure is met. It's as simple as that. In this case, he wants £15m it would appear, so you'll either be paying £15m or he won't move. It's just how the club operates.
Hope this was interesting/useful.