With no independent grading, pictures revealing next to nil in most cases and that mind boggling range of mostly crud quality... wouldn't trust my uncle if he was selling pearls on Ebay! I did get some... oddities but do not have any secret recipe for getting your money worth on FleaBay, nor I believe there is such recipe. If you know the seller from elsewhere, yeah... alright. But that is beyond the point here.
As for Ebay's ratings, it really is mind bending how can anyone put much value in it: every seller seems to have next to flawless ratings, yet Ebay sales have the darkest reputation of any Flea market and even legislators had to agree that no one can enforce any standards there. Those ratings are engineered, faked and bought every day... don't even bother. Well, at least that's my 2c and what I do.
Basically, have yet to find any tempting offer of round South Sea bead nucleated pearls on Ebay (maybe I didn't look much...). A few for baroque's seem half tempting and for relatively low prices might not be much of a gamble (say, &100-$150 or so). Why I tend to look for baroque ones... because I prefer them anyway, but also because these appear less in usual jewelry stores which makes them both harder to find and more likely to appear in alternative sales online, Ebay included.
High quality round South Sea are hard to find anywhere... so I would not expect any in a discount venue like FleaBay, nor there seem to be many. This being said, it is still a gamble. And no, I don't believe high prices on Ebay guarantee anything about the pearls either - just prove that the seller is bold!
My 2c
For what it is worth, Ebay looks like a shortcut but buying there looks like professional buying: what, say, a jeweler would face taking a trip to China or Banghock to buy pearls and precious stones directly (which relatively few do anyway). And the better merchandise never makes it online anyway, it goes into the established trade pipeline. Some serious seller have Ebay shops too for advertising, some work... some not. But it is definitely NOT a revolutionary market where everyone has nearly perfect reputation LOL, like those ratings would read taken at face value. Those ratings are as good as buyers are good telling good from bad - and that still comes down to the professional buyers that don't go Ebay anyway.
Sorry for the long post... There is a whole forum here about Ebay misses and near misses. Take a look, if you wish.