I'm not opposed to selling prospects for rentals, so I hope no one takes this post out of context, but I am cautious.
A farm system exists to supplement the big club, and that means trading away future value for present value.
However, there always seems to be some people that are 100% about instant gratification and want to sell off the farm seemingly at any time. That's all fine and Dandy, but keep in mind NOT selling the farm at times has given us Bogaerts, Devers, Vasquez, and Barnes and in years past the likes of Betts, Pedroia, Benintendi, Lester, Papelbon etc.
And yes, sometimes you trade away those prospects for the likes of Sale, Lowe, Varitek, etc. But a good GM doesn't just buy for the sake of buying sometimes you the best trades are the ones you do not make.