You misunderstand what I am saying. We know we can pull the information I said we could. What I am saying as my reply clearly stated in my last reply was to do something like you want since the user info exists on the forum. You first have to issue a query to only get userids in the products table and then issue a query on the forum database to get there email.
That in your case is 421 queries for that function. See in a standalone classifieds where everything is in one database I can issue one query to the products table with a left join on the users table to get the email. See what I mean now.
I can do the same thing even with the forum user table if things existed in the same database. However the forum tables is not always in the same database as the classifieds tables and the query to left join whatever forum user table would be different in every last integration there is.
What I am saying is I can not include something in the application as standard if:
1. It would not work for everbody
2. It would create too much server overhead.
I can do something like you want for a standalone classifieds thats easy. If I can think of a way to do it for forum integrations I will see but do not expect some answer for 5.3