My tip: set up some saved searches for more general terms, and look for mislabeled items, where the listing title is for a less expensive item (and is priced as such), while the picture shows a more expensive item. Just ask the seller if it's the actual picture to make sure that they didn't just copy-paste the wrong thing from google images. A few might catch on, but most will just say "yeah I took the picture myself".
e.g. it's easy for a seller to mix up CRS326-24G-2S+ (~$200) and CRS326-24S+2Q+ (~$600), and price it as the former.
Also look for bundles. Maybe you buy a complete server just for the chassis, because it's the same price, but now you've got some spare components you can sell. Sometimes a bundle (like a fully assembled or barebones system) is cheaper than the single part, because people tend to search for single parts rather than every possible bundle. I got a NIB CSE-745 for $300 because it was being sold as a barebones system.