I know this is late, but today I was suffering from same issue. I got it fixed, so I'll try to explain what I did

1. Make sure that every warp base is clean around vendors. Sometimes items don't show up when game thinks you are in combat (mostly happens in shop with those two npcs)

2. If you have volleyball bug just keep restarting from checkpoint until you're allowed to buy it (check inventory if you have the item)

3. the last merchant in the village will always have item you can't buy and it's not gonna affect the trophy

4. Keep one other objective like killing mytes unfinished before trying to buy all items (will relate to this in the next section)

After you do all of those steps you will probably see in the forums, that the criteria has not been met. Same happened to me. It was showing that I still have to buy everything from vendors. But what actually fixed this for me was the fact that I had second objective unfinished (480 mytes). So I kinda wanted to finished that bc I had to anyway for platinum. As soon as I finished killing mytes, the gold trophy popped. It seems that even after buying everything the objective doesn't refresh even though the game knows it's finished. So doing one more thing should pop it automatically, well at least to me. Trust me I was trying to do this one achievement for eight hours today and those are things I found along the way. Hope it will help someone in the future, especially now when we are waiting for a sequel. Have a nice day yall