To expand a little more.
A lot of those high end artifacts have been around in collections for decades. They are usually traded and bought between those collectors, and often times owned several times over by the same people. While it's true that collectors with deep pockets pay astounding amounts for some pieces, there are lots of personal find collections there, my friend John Beasley having one of them. He's one of the most prolific creek walkers out there, and the stuff he finds is amazing. He's only recently bought a few outside pieces, but chooses to show his personal finds. My favorites displays are usually the personal find collections, and I really enjoy talking with those guys, and they are usually eager to share and appreciate the interest.
While a lot of hardstone, pottery, and later period flint pieces were found in mounds years ago, there are still fantastic flint pieces being found today, like Paleo and Early Archaic types that are my favorites. Those types are not found in burials or mounds, but are usually random finds near known occupation or lithic manufacturing sites.
I know of some fantastic pieces found every month, but the finders log many miles looking for them, and they are as high end as any ever found.
But yes, some collectors do a lot of trading, buying and selling at those scheduled shows, as it's often the only times that they get together with other collectors in such large numbers. They are also very welcoming to new collectors and hunters. Just like deer hunting, a significant number of them are aging out, and really hope to pass everything on to younger generations.