No, I wouldn't sell him, and it isn't to do with the price.
I thought at the time, and still do, that £80 million was expensive, but that was going to happen with whoever we tried to buy - everyone knew we were intent on bringing in a new CB so the market pushed up the price for anyone who fitted the spec. Comparing his price with van Dijk's £75m makes him look expensive, but we were never going to be able to say, "we want to pay less because van Dijk cost Liverpool less." Selling him for £80m, if a buyer made that offer, means we would break even on the deal. That's it. We would still need a new CB unless we revert back to Smalling, Bailly, Jones etc, or would pay the same or more for a replacement.
I would agree that he doesn't shout "world class", although he compares well with CBs in the rest of the EPL, with one or two exceptions (accepting the EPL is light on top CBs at the moment). He is a leader in a young team, though, which is why Ole handed him the captaincy so soon (and I suspect that was as much a consensus of the other players as it was a decision by the manager), and that is valuable in itself. He's also in his first season at Utd - being previously at an EPL club helps, but you have to allow for some settling in.
In particular, it shouldn't be underestimated how disruptive it would be to change our main CB a year after we brought in a new one, and change our captain, at a time when the pieces of the jigsaw look as if they are beginning to fit.
The big temptation, given all the media coverage, is to go for Koulibaly, probably the only CB in the same price range who is an upgrade and probably available - if that happened, I would ideally see him as a player additional to Maguire, not a replacement. It would probably mean getting a couple of other CBs off the books. His price would probably go up if we went for him , and you wonder if that has been the sticking point previously, as we've almost certainly pursued him. We're not leaking goals at the moment, though, and you wonder if it's considered a priority for the club just because Liverpool and a clutch of other clubs are said to be after him. If money was no object, he would be a great signing though, and would give us the satisfaction of denying Liverpool an "impenetrable defence".