If you're reading this then congratulations!  You're doing your homework!  And seeing as how you've probably been to a number of resources before you clicked in this guide then I don't have to tell you what you need to look for.  But I will sum it up, just incase.

The ideal paintball mask for just about anyone has a 180 degree view (or better),  has decent ventilation while protecting your mouth, and most importantly, has a thermal lens.  No matter how advanced a single pane lens system is, you're stil better off going with a thermal.  For the added $10 it will cost on most masks, it pretty much guarentees no fogging.  A great single pane lens may not fog up often, but when your out on the field it may be that rare occurance that ruins your whole day.

One thing I cannot stress enough when your shopping used items on eBay is to look at what your buying.  No really, LOOK at what your buying, read every single word of the listing 3 or 4 times.  Go over every picture in the listing at the largest resolution you can get it.  If your spending your hard earned money on something, make sure you know exactly what it is, and approximately what condition it is in. I recently picked up a DYE Invision mask system, I was looking for a thermal lens, saw the high end goggles, and blindly thought "yeah! Sure! They've gotta be thermal!"  After I bid, won, and paid, I realized they weren't.  Now, they're great goggles, they came at a great price, and I can get a replacement thermal lens for them, but I was careless and didn't get exactly what I was originally shopping for.  Learn from the mistakes of others!

You've made it this far, now it's time for recommendations.

The JT nVader is a great, affordable mask system, the Empire eVents are nice aswell.  All of Dye's goggles are fantastic, they consistantly make great high end products.  The Proto Switch FS!!!! NOT EL!!!   are also affordable, and effective.   Beyond that, check out gear reviews at www.pbreview.com or www.pbnation.com (great forum there).