It keeps saying that I might have a trade ban and to check my inventory!

As the message says, you might have a trade ban or your inventory might be set to private. Make sure that your inventory settings are set to public and that you haven't done anything which could trigger SteamGuard's protection. Things like changing your email, changing your password, logging in from a new device or browser, logging in from a browser after clearing your cache and cookies and things of the sort trigger such protection.

If it's because SteamGuard, you won't be able to trade for a week.