Hi! May be it's crazy question, but ;)
Where in my acount under Cpanel located MySQL bases?
In what folder?

I'm not sure I quite understand your question but here goes anyways.

If you're looking to set up a database in cPanel under the customer interface in cPanel all you have to do is look for the MySQL logo and also it says MySQL databases. If you're looking for where the files are kept that hold your database then you're out of luck. You do not have access to those.

OK I mean when I want move acount from one reseller to other how movin DB?

You can move your database through phpMyAdmin. You'll find the link for that at the bottom of the database managment page. Open it up and chose the database you want to move. You'll see a tab up on top that says "Export". Make sure you select all the tables and check off data and use complete inserts to be safe. Check off "Save as File" and then hit GO. You'll also see an option for compression and you may want to zipp or gzipp the database if its large. The only problem with this is that if its a large database, when you go to restore the database phpMyAdmin may timeout due to restrictions set on how long a PHP program can be run. If that happens don't be afraid of asking for help from your host.