I try very hard to credit freebie creators.  I assume alot are providing things as freebies to get thier name out there, so I feel obligated to do my part.  I also try to send a note to people if I use thier freebie in my work. - Just so they can see it -

Merchants I don't though.  The list is too long to keep track of with just freebie people.  I have let merchants know how much I like what I have gotten from them before though.  So far few have ever responded (two were Richabri and Francemi).  More merchants should write on thier product pages that they would be interested in seeing renders done with it - then if people know the merchant is even interested they will let you know.

I try to keep track of venders who I notice saying things in threads like this. 

I am:  aka Velocity3d