Interesting read here. Seems like miles, because they are somewhat intangible ppl have all kinds of ideas about it. I don't buy miles but I do buy gift cards. If I purchased a Dell Gift card from someone and didn't use it within a specific time frame I would consider it out right stealing if the seller tried using it on his own even if it's about to expire.
Also I wonder if you can use זוטו של ים before it actually expired. Until that point to when it expired it still usable by the buyer and should still be his. Just because the seller still has access to the item he sold and it's being stored by him that shouldn't change the dynamics.