You can start with an eBay store. eBay stores can be started for $16/month, $50 or $300 a month. Amazon stores could also be used if you get t high enough volume.

You can also set up your own website and ecommerce site. This will come at a higher cost initially but probably lower overall monthly cost and you control everything. So if you start on eBay think about transitioning to your own site soon.

Advertising on google is us to you. You can spend as little or as much as you want. It depends on the volume of traffic you want and your budget. The better your ad performs the lower your cost. Look for google.com/adwords to get started.