I believe that these short term sales are mainly designed as attention-getters for the vendor and his products.

Well, it won't work, for everyone not following those sales because (s)he isn't able to be online when Renderosity decides just deletes the "Exclusive Vendor Sales" newsletters. Why would I read them when I can't use those sales anyway? Just to waste my time, or for the pleasure to know there will be an opportunity I'll have to miss?

It's more a "feel good" feature for Renderosity, probably so they can say they do sales without losing too much money by having actual sales. And yes, there has been a longish complaint thread about that right at the beginning of that fad, and Renderosity just replied something along the lines of "tough luck", so don't bother complaining, it's not like your opinion matters.