DISCLAIMER: I am not a pro modder, but I figured out how to make this work on my 3.6.8 copy. Back up everything related to vb including files and database. Make these modifications at your own risk. NOTHING IS GUARANTEED.
With that said, I just upgraded my forum from 3.5.x to 3.6.8 and the following solution worked for so it should be a good solution for those with 3.6.8 that have no idea how to restore itrader functionality.
The reason it does not work is because somewhere in the upgrade to 3.6.8 the postbit_legacy and postbit templates are wiped and they lack the itrader template conditional which displays the itrader values for your members in the detail below their avatars.
Go to your admincp>>Styles & Templates>>Style Manager>>Find the template you wish to modify and click the "<< >>" which appears on the same line in the last column.
Scroll down to Postbit Template. Double click Postbit Template. If you use Postbit Legacy (check your vb options), then double click postbit_legacy, search for the line that contains an if condition for joindate, i.e. search for "joindate".
Hopefully that will lead you to this text:
PHP Code:
<if condition="$post['joindate']"><div>$vbphrase[join_date]: $post[joindate]</div></if>
Paste this immediately after it:
PHP Code:
<if condition="$post['userid']"><div>Trader Rating: (<b><a href="itrader.php?$session[sessionurl]u=$post[userid]">$post[itrader_total]</a></b>)</div></if>
Save and your trader ratings should now function in 3.6.8.
If you use the postbit template, you can follow the same steps for that template but I cannot guarantee you will get the placement you want.