I'm wondering if there's some tutorial or program out there to instruct me on creating pose files to compile and sell in the marketplace. Any help would be greatly appreciated. Thanks.

Assuming you can create the poses, and if not it's way too soon to be thinking of selling, just create a new category in your Pose library (at the bottom, or is it top, of the pop-up list of existing folders in Poser 4) and then with your figure selected hit the plus sign at the bottom of the library bar to save a pose to the library. Gather a set of poses and you have a product, remembering: if you need to move the figure as a unit, for interaction with another figure say, turn off IK and move the Hip - changes to the Body aren't saved. use P3DO to make PNGs from your RSRs if you are using Poser 4, or RSRs from your PNGs if you are using Pro Pack or Poser 5. make sure the pose names reflect the figure used, and ideally the nature of the pose: 001.pz2, 002.pz2 etc. is unhelpful.

Really? I thought doing that would just save the figure with props and stuff atatched, and not only the pose itself to aaply to other figures. I'll give it a try. Thanks for your help.

In the Figure library you get the whole thing, figure, pose and props - the Pose library saves poses, the Face library saves just the head morph dials, the Hand library just the hand settings and so on.

Any advice on making thumbnails?

There are better methods, but mine (second-hand) is: get a square image you like and render it, Display>Paste onto background, slide the camera forward so that your actual elements are behind it and all you can see is your rendered background (Poser will switch back to unrendered view), save your library entry. When you save anything to a library Poser uses the current working view for the thumbnail, so this way you get your image. Display>Clear background image then leaves you free for the next one.

thanks.