Paul,
The history of beads is filled with examples of copies which are now antique.
Going after antique beads (or antique anything) has been discouraging for centuries.

If you feel you have the aptitude -and fortitude- go for it.
Some people are willing to suffer. "No pain, no gain."
It's like gambling. But you get to keep your chips.
Sometimes your beads will turn out to be valuable.
Sometimes your beads will become lessons well 'earned.

Just Fred

P.S. Please make pictures of some of the dealers in Chatuchak.
I am sure we would all enjoy seeing these images posted here.