Potential resale value of a player means nothing to me. When you sign a player the intention is to keep them for as long as possible, or as long as you need them. When you get to the top, you want to keep players till they retire or you think they no longer offer the club value for the wages they're on. Man Utd could've proabably got an insane amount of money for someone like Giggs 6 years ago. But the better option is to keep Giggs till he drops or leaves on a free for what he brings.
Phillips left at a bad time for the club. Essentially a fire sale, and only with hindsight can anyone say we should've sold him earlier. What if after the two 7th placed finsihed we broke into a UEFA Cup spot? Quinn wouldn't then say in 2011 that we should've sold Phillips. Only because of our trouble after the second 7th, and relegation with being a place for bargain hunting clubs could it be suggested we should've sold him early.
Being rather long winded here because I'm just not sure I'm getting my point across correctly