Retrodevil,
I think it's commendable that you took the trouble to ask for opinions on this, quality in the marketplace would improve immediately if more new vendors followed your example.

Some knowledgable folks have given good advice so far, I'd just like to add my pet peeve, and that is efficiency. As we know Poser doesn't handle big scenes easily, every item we add to a scene worsens the problem, high polygon counts and large texture files are the enemy.

I downloaded your freebie models to check out the meshes and the textures and there's room for improvement in both areas. The mesh on your Small T is denser than it needs to be (and quite a bit denser than Vicky's corresponding bodyparts). Most of that density isn't contributing anything to the product. You could optimise the mesh a lot, either cutting out unnecessary polygons or rearranging the polys to give some thickening at edges, better creases etc.

On the helmet there's an area of incredibly dense mesh on the 'cheeks', all it is doing is increase render times with no benefit. You could rearrange the mesh to better define the shape of the visor opening.

The textures are HUGE, they'll gobble up RAM with NO benefit to the final image. As a rule of thumb when deciding on texture sizes, think about the largest size the textured surface (eg: helmet or notice-board) is likely to be in a typical finished render. So if a large-ish render is, say 1600 pixels high and the object might take up half of that height in a close-up shot, that's 800 pixels. There's no benefit in applying a texture bigger than 800 pixels high to it, it'll just be downsampled at render time. If the textures are intentionally blurry (eg: your helmet), then they definitely don't need to be so big. If you think closer-in shots might be called for, then make sure the mesh stands up to close scrutiny (if the mesh shows crappy facetting up close there's clearly no value in high-res textures on it).

When deciding on mesh density (how many polygons you should budget for a particular part of your model) you should look at its importance in the overall scene. Make every edge and every polygon count. For example the push-pins on your notice-board should ideally be no more than about 30 polys each (I don't know what they are currently) because in most renders they'll show up as no more than 3 or 4 coloured pixels. Using a few extra polys to bevel the frame will really improve the render, whereas extra polys in the pins would be largely wasted.

This may sound anal but I think it's worth cultivating an attitude to enonomy of means early on. In some items like realistic hair props high poly counts and big textures are unavoidable, so we all need to avoid bloat wherever we can, particularly in 'background' stuff.  When you get into making more complicated props and scenes that attitude can make the difference between a useful product and an unusable resource hog.