I would agree with others that the best use for purchased poses is as a quick starting point or, if you are going to have a number of background figures, to save time on the ones the viewer won't be looking at too much. Having said that, sometimes you really do need to do something fast and a canned pose can do the job if it's of a good enough quality - as Blackhearted has said, quality over quantity every time.

I find that IK is a great boon for getting a little dynamism in a pose but for a seated or standing figure, I prefer not to use it. Instead I decide roughly how I want the figure and then start by positioning the hip. I then work down the legs (sometimes tweaking the hip if it's clear that I need to) finishing with the feet, getting them where I want them. After this I work my way up the body to the shoulders and then down the arms to the hands. Next I deal with the neck and then the head. You can use references (I've used a few old comics lately) but I find that they are of limited use becaust the furniture you have in Poser is not going to be quite the same as that in the reference, by doing things this way you will usually end up with something that looks natural to that chair because you are building the pose around it.

This done, I decide on the camera position that I want for the render. Once that decision is made I can start to tweak the pose, usually spending time on the hands - but only if they are visible, there's no point in perfecting what can't be seen. I try to pay attention to what is phisically possible for a joint to do but at the end of the day, if it looks right from the camera position that I have chosen, then I don't mind breaking a few rules/bones.