It depends on what coin you are going to buy. However, of course, we only buy coins that we think do have potential in the future. You may purchase them once it gets listed. Most of the time, ERC-20 tokens are being sold at a meager price in IDEX market. Then you'll have to wait for it to get in big exchanges because it will surely pump once it gets listed.
Most of the tokens, the prices on IDEX are not very good, and there are a lot of alligators who choose to trade there. So if you choose a new coin, you have to worry about whether you can stick with it.