In the Nexomon world, every city or warp point has a sort of vendor - each with their own sort of stuff to sell. Usually these are near the warp stones.
This achievement requires you to spend 50,000 coins on them. If you are following the story, trying to catch and tame most Nexomon you see along the way, you should get this achievement before the end of it.
How to earn coins in Nexomon:
Winning a successful battle, be it against another Tamer or even against wild Nexomon.
You can also earn coins on trades and quests.
If you intend on farming more coins, you can attach one or more Greed Cores to your fighting Nexomon as they can be stacked and increase your Gold gain after each battle.