I was wondering if anyone knows a good place/market in Bangkok where people are interested in buying some old, collectible Thai bank notes which are 20-30 years old?

Have a look at this site

 

http://www.eurseree.com

 

They've been around many years and I have bought stamps from them before.

 

Their next sale is in a few weeks and they usually hold them quarterly.

Thank you for the site. It looks interesting. Seems like it is for auctions though. I don't have that many bills, maybe only 6 or 7 and I was hoping to just sell them cheaply to someone who is a collector or a dealer who wants to resell them for more.

There is a section in Chatuchak weekend market that deals in old currency, coins amulets etc.      Maybe give that  try.