The Location directive takes the "path" relative to the URL root. So if phpdealer.com is the URL, and you reach your SVN repo by browsing to phpdealer.com/repo then the Location needs to be "/repo" not "/var/www/vhosts/phpdealer.com/repo".
Locations are from the URL, Directory is from the hard disk.
So for example:
<Location /repo>
DAV svn
SVNPath /var/www/vhosts/phpdealer.com/repo
AuthName "phpDealer"
AuthType Basic
AuthUserFile /var/www/vhosts/phpdealer.com/repo/conf/passwd
Require valid-user
</Location>
<Directory /var/www/vhosts/phpdealer.com/repo>
Order allow,deny
Allow from all
</Directory>
Hope that helps.