In paleontology, "amateur" can be
From the very beginning, amateurs have been essential to the development of paleontology. Nineteenth-century scholars trained in anatomy and geology like Georges Cuvier, Richard Owen, and William Buckland did much to set the academic foundations of paleontology, but their efforts were matched by tireless fossil collectors like Mary Anning. Not everyone could be a professional academic, but through the 20th century, other fossil hunters like Charles H. Sternberg and Barnum Brown continuned the tradition of careful collection that Anning had started.
It is true that Anning, Sternberg, Brown, and other non-academic paleontologists sold their finds. The life of a fossil hunter was not an easy one and much of their income came from selling the fossils they found. How is this distinguishable from modern-day collectors who dig up bones for the auction block? There are at least two main differences. The first is that fossil hunters like Anning, Sternberg, and Brown were often commissioned by academics to find fossils or they sold those they had already found to museums. Modern commercial collectors do not show the same inclination and are often more concerned with how much money a specimen can make. (As the fellows at SV-POW! remind us, this can keep new and significant specimens beyond the reach of paleontologists.)
Secondly, non-academic paleontologists often keep detailed notes on geology, the position of the skeleton, and other important factors that places fossils in context. It is not enough to simply remove a skeleton from a ground. The very rock it is embedded in is part of its story, and amateur paleontologists often play a crucial role in collecting this sort of data. Commercial collectors do not always show the same concerns and so some of the most important information about a fossil, like where it comes from, might be lost.
Non-professionals have always been, and will continue to be, very important to the science of paleontology. There is always more work to be done than can be accomplished solely by those who have Ph.D.s and there has been a long tradition of self-educated people who have made important contributions to paleontology. The divide these days is not so much between amateurs and professionals as it is between those who care about science and those who want to make a few bucks.