Author's NOTE: totally not mandatory to read but might give you some closure or insight as to what I was thinking when I wrote this.
Who did Harry buy the ring for? I don't think he even knew. Deep-in-the-closet Harry carries on an affair with out-of-the-closet Draco. Their relationship has always been complex. Draco loathes Harry's moralistic conventionalism- and in a moment of vulnerability, Harry decides that he is going to throw his arms up in the air and declare 'checkmate!' By asking Draco to 'marry' him- its his way of telling him he's ready to come out, but for Draco- it's not only too little, too late, but the deeper hurt is the way Harry applies the idea of 'marriage' - which Draco in hostile resentment deems as not applicable to his identity.
Like I said- their relationship is complex. They hate/love/need each other, and yet at the end, realize how unhealthy this is. So Draco rejects Harry (like he feels Harry has rejected him all along by denying his own identity) and Harry, drops the ring in the end. There are few different interpretations as to why. I'll let you decide.