If I list something on ebay, and it sells that day, it won't show up to match inventory to the sale. What can I do about this?

Since the listing has been sold by the time new listings are pulled, it's not getting pulled via the integration to be added to inventory. One option would be to run a manual pull from eBay once you list items so that they can be pulled in before they sell. You can do this by clicking the profile icon > Integrations and clicking the refresh icon next to the eBay integration.