With designer resale sites having a bigger moment than ever before, we tapped the pros at The RealReal for the latest intel on what's actually got the most promising return-on-investment. We know, "ROI" sounds like a term that should only be brought up in a boardroom, but it actually applies to your closet, too. To really make a big-spender purchase you won't regret, it's worth doing a little research to see what types of pieces will actually going to keep their value — or, better yet, appreciate over time. Because, honestly, there's nothing worse than finding out the item you spent your rent on a few years ago isn't even worth a few-hundred bucks today.