I recently received an email from a person who couldn’t afford to buy a new mic or mixer, and was thinking about buying used equipment on eBay.
I would be wary of buying used microphones on eBay … People tend to hold on to microphones until they start to deteriorate, and then sell them. Depending on what you’re doing, my suggestion to you would be to not economize on both a mixer and a microphone, but to get a decent shotgun mic now, and then a mixer or stand-alone recorder later. Without the amplification from an external mixer, the audio recorded by a DSLR won’t be as clean, but I think you’re better off buying a microphone that you’ll be able to use for years, then spending almost as much on a used mic that’s going to crap out on you in short order.
One big warning though: DSLR cameras do not provide phantom power, so many shotgun microphones simply will not work when plugged directly into the camera. You have to make sure to buy a microphone (new or used) that will accept batteries.
I’m a big fan of Audio-Technics microphones, because they provide excellent value. Here’s a video (not done by me) that compares three different low-cost AT mics. This is a more detailed version of what I would tell you myself.
If you decide to buy a used mic and mixer to avoid the phantom power issue (virtually any mixer will provide phantom power), I would suggest looking for any Shure mixer that’s in decent shape. Three inputs are fine, and whether the meters are analog or digital doesn’t really matter (it’s more of a style choice than a functional consideration).