I'm looking for a legit place that sells gold bullion or gold coins in Chiang Mai.

Any Gold shop.Most Thai buy those little 5 baht ( baht size ) bars .I think all shops have to sell or buy at an official rate of the day.You will need to bring your Passport.Thais use their I.D. 

Gold Bullion best bought at Aurora Shop Central Airport plaza, Central Festival or Promenada. Easily sell back should you choose to. Small margins between by and sell price. Coins not sure. 

1 hour ago, tigerbeer said:

Gold Bullion best bought at Aurora Shop Central Airport plaza, Central Festival or Promenada. Easily sell back should you choose to. Small margins between by and sell price. Coins not sure. 

and keep your receipt.

Warrorot Market - several stores

5 hours ago, tigerbeer said:

Gold Bullion best bought at Aurora Shop Central Airport plaza, Central Festival or Promenada. Easily sell back should you choose to. Small margins between by and sell price. Coins not sure. 

Thai gold or Swiss standard bars(Swiss, Australian, ....)?

 

Never heard of coin trader in recent years.  One tried it about 10 years ago and closed.

Try...ANY gold shop.