Cross listing on multiple sites lets you utilize their the SEO and show your product at the top! Thus enabling you to sell more! Let's look at an example:
How eCommerce sell their product
eCommerce sites pay a lot of money to make sure that when someone searches for a product their listing comes up to the top. If you cross list on more than one site then your results will appear from all those sites on Google.
Lets see an example
We originally listed a shoe for a client on eBay. Check out the image below.
After finishing this listing we right away listed the shoe on Mercari and Poshmark.
Soon after posting we checked out Google...and viola!
Now DailyRefinement controls 3 of the top search results and is the only one to have brown shoes appear at the top!
You can see pretty easily using this example how cross listing can help you grow your business.
So why doesn't everyone do this? Because listing and cross listing takes up a lot of time! What you should do is focus on sourcing good products and automate away the cross listing process. Just go on this link and get some free cross listings done to see how easy cross listing can be.