So I've bought and played with products for many years, and never really considered creating content for sale, but I am beginning to consider doing something. At this point I still don't have a plan of attack or even exactly what I would like to produce.

I am wondering though, If it is appropriate to find a template and create textures for some of the existing products. What would be the best approach? Seek the creators permission? Can someone share some thoughts on how to go into merchanting in this manner? I would not want to create something and attract a firestorm of hate or disgust. This really would be a hobby, and I realize for some of these vendors this has become a lively hood.

If you would like, take a look at my store and see if there is anything there that interests you. If there is, send me a sitemail and we can discuss it.

If you buy an item that has a template file, you can use the template create content for that item.
To SELL the item you need have a quality item, that is packaged correctly, and meets the minimum quality requirements of the site you intend to vendor with.

I too am curious about becoming a merchant here. I have a few ideas for feasible products. My starting questions would be: i) is there a requirement for regular production of new content (i.e. will the production of only three items a year be frowned on), and ii) what is the % cut that Renderosity takes from smaller and less prolific merchants (I assume a sliding scale)?

no, no minimum requirements.
Renderosity takes a 50% split from NON-Exclusive Vendors
45% from Exclusive Vendors
and progressively less from the 30 Top Seller. Top Sellers generally upload at least once a week.
Please see https://www.renderosity.com/how-to-become-a-vendor-at-renderosity-marketplace---all-the-info-you-may-need--cms-19332

Many thanks, Donnena. I've sent it over to my Kindle for reading in due course.