Simple answers for simple problems -

Three quick options that can be used solo or together.

For Amazon, you have to get in the Buy Box on the product detail page. Also, having a high amount of positive feedback and the lowest price helps - as well as shipping out for Prime customers.

1. Get a repricing script, there are lots of vendors out there. Set your ceiling and floor pricing and then reprice to be under your competitors' price.
2. Setup an inventory management solution online - that will integrate with other multiple marketplaces. Think fulfillment by Amazon (FBA), eBay, Rakuten, etc for your solution when choose something like Lettuce or UnLeashed.
3. Setup an E-Commerce store through a Shopify or BigCommerce

If you are selling a ton - you could have FBA manage fulfillment. I have helped a number of power sellers online so if you need more help, I will have recommendations --