Personally, I wouldn't want a trial as much as I'd want the seller to take me on a hunt to see how the dog works for him.

My thoughts exactly. Also, I would want to see the dog hunt ALONE. Many a "me-too" dog can look really good paired up with an independant dog they are used to following.
One of my dogs I traded for took about a month before he would get 20 yards from me but I had seen him hunt with his prior owner. Then one day he left. He still has his moments, but more times than not you better have a good set of batteries in the tracking system when you unsnap the lead. If I were to ever sell this dog on "trial" I would guarantee he would be brought back time and time again. Some will go right to hunting, some take more time, and then there are those who may never hunt for someone else. I don't think any of these make a dog any better or worse...it's just their personality.
As far as being advertised as "finished"...what makes this so difficult is that everyone has their own opinion of what that means. I honestly feel that a lot of people (not picking, but especially new-commers to the sport) have never had the chance to hunt behind a TRUE powerhouse, so opinions of "finished" can vary greatly.