This achievement was glitched for me in a positive way. I only did 3 things and the achievement popped for me of the 5 it says you need.
If you want to check out all the progress towards all of the belt buckles you can hit
and go down to Progress. From there go to Awards and it will lay out all of the available belt buckles in the different categories.
The ones I did was the 2 under the Travel section (ride from Colter to Adler's Ranch and Ride from Valentine to Strawberry) I went to a general store and bought a couple horse tonics to refill my horse's stamina when it was empty. I had a timer going and they were not difficult to get there in plenty of time
The other one I did was in the trade section. Go to a butcher, doctor, and fence to sell them one item. I used an herb that I picked up around the game and sold it to the doctor in Valentine. I killed a random animal and skinned it and took it to a butcher in Valentine as well. Lastly I went to the fence in Thieves Landing and sold him an item I picked up after looting enemies.
Achievement popped after the third gold belt buckle I got.
Edit- Looks like the achievement might be glitchy in a good or bad way. Some people are reporting it taking the 5 buckles or more to track/unlock. I will report some more easy buckles later when I have some more time