You will note that almost all of my coins are certified by either NGC or PCGS, although I do buy a number of "raw" (uncertified) coins and then submit them for certification myself. For the new collector I highly recommend buying certified coins, and from well established dealers or auction houses. I can only recommend NGC, PCGS or ANACS as certification companies, unfortunately there are a bunch of other companies offering these services which knowledgable collectors will be wary of.
I don't recommend buying anything "raw" off ebay, it's a good way to lose a lot of money. Be especially wary when the raw coin comes with a good story, found in my grandpa's attic/inherited from a rich uncle/etc., almost always a sure sign of a fake or heavily misrepresented coin. Many of the older Chinese fakes were pretty obvious, but they have gotten much better in the past few years and have been able to fool even experienced collectors. And the Chinese have even been faking PCGS slabs, and doing a reasonably good job of it.
Take the population numbers with a grain of salt. The NGC and PCGS numbers are pretty good at giving you an idea of scarcity, but keep in mind the great majority of European collectors don't have their collections slabbed. It is useful to compare the numbers graded though to older reference material, such as Peck, ESC or Spink references. If they list a coin as rare or extremely rare, and only 1 or 2 have been graded, then it probably is actually rare.
There are very few English language reference books for German States coinage, I still use my Standard Catalog of German Coins 1601-Present on an almost daily basis though. While the pricing they show is long out of date by the time they update it you can get a sense for the rarity or collectibility from it, and it is still one of the most comprehensive listings available.