The answer to this question, as with selling any player, should really be based on whether we think we could end up with a net positive (in terms of footballing ability in the squad) for the money we would receive. Think about when Michael Edwards sold Coutinho. It was seen as the end of a bright period and a massive blow to the club - it was actually the beginning.
It's widely accepted that we have been poor at selling players. We have old-fashioned, romantic ideas. We're also too hung-up still on the idea of individual brilliance/ability. I used to read stories about PSG and Barcelona being interested at prices north of £100m. I would have sold at that point. It's no secret that I never rated Rashford as a top footballer and knew the odd flash of magic or purple patch of goalscoring masked major deficiencies and technical flaws.
For £100m+, we could definitely have got a couple of players who really would have made a huge difference to our XI
Nowadays, what would we get for Rashford? £60m, maybe? I'd say it becomes a more difficult decision. I quite like Rashford as an impact-sub, and he is better than he has shown lately. For me, if a team were willing to pay £80m+, it's 'sell'. If not, keep, play back into some form/confidence, be realistic about his strengths and weaknesses and use him as an impact sub, coming on after 60/70mins to utilise his directness and pace