Buy from a shop called Tang Hua Seng - they have about 5 branches in Yaowrrat (and one in Seacon Square) and Bars bought there will sell more easily when you need the money back, they have an excellent reputation among Thais; often in Yaowarrat you will see other Gold shops completely empty but Tang Hua Seng branches are packed with Customers.
If I recall correctly Bars are 1, 2 5, 20 and 50 Baht weight (50 is the biggest I have bought anyway).
You will have to pay Cash - and you will be amazed at the huge bundles of notes being paid in!
You will take collection of the physical Gold Bar - no receipt is given.
Patrick