[HowEver]

I'm going to be selling a fantastic MacBook Pro 15" (2.6Ghz Quad Intel Core i7, 500GB SSD, 16GB RAM, new top and bottom case, AppleCare, etc.), and wondering: would a buyer want Mac OSX 11 El Capitan, or should I install Mac OS Sierra?

 

[rgray]

Why not offer the buyer a choice?

A high proportion of potential buyers probably don't have an opinion, but those who do will/may be pretty firm.