old advice for selling, is it still valid?

You realize there is probably no "good" answer to your question(s)...(?)

Here is my 2 cents:
There are only 2 opinions that matters:
The opinion of Mr. Supply and the opinion of Mr. Demand.
Everyone else's opinion is just a theory.

I see only 2 tools we can use: the ancient "trial and error process" and the "common sense".
You change something, you wait, you try to interpret the feedback you get, you make new changes based on the feedback and your interpretation and you repeat the whole process again...

The Mighty Gods of Marketing and Data Mining don't seem to serve very successfully their blind disciples and followers.

Usually models have a number of clients, if price is set lower it stays same amount of clients, if price is set higher it also stays same amount but only to some degree. This so called pricing sweet-spot is something that depends on meany things, for example the uniqueness of the model, the need for it and how meany there are of it (competition), lots of things to consider + trial and error/experience and then one gets to develop some form of gut feeling about price he can set. I do not believe number of models has big impact, models that people can use and want to buy that will have biggest impact. I have a relatively small library of 53 models but in comparison get more sales then some here that have over 300 models.

Tip: stay away from competition, only get involved in a crowded category if average prices are high and you are convinced you can bring something better for a bit higher price.

I think it depends a lot.
If a user has 300 models, he will not necessarily sell more than one that has 30.

It depends on the offer and the demand, if the user with 300 models has only models with a specific theme (for example: ancient Chinese sculptures), simple presentation and little variety of formats. I doubt that it sells much more than the other user who only has 30 models but with very varied themes (characters, clothes, vehicles, animals, weapons, etc.) and more variety of formats and better presentation.