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It's not a fine line in your head. You know what you are doing.
From a practical standpoint, buying an ad on a niche site is a fine and sensible thing to do. Buying a link on an off-topic site just to game the system is gaming the system.
On the flip side, however, I don't think you should limit yourself to only buying advertising links from low-PR sites! LOL
I think it makes perfect business sense to consider the traffic levels and PR of sites you are considering purchasing advertising on as part of your decision on where to allocate your advertising budget.
If you are considering purchasing advertising on one of two sites, which are otherwise identical except one's a PR2 and the other is a PR8, I don't think you are cheating the system if you decide the second site is a better use of your advertising dollars.
However, I would not advise buying advertising on sites promoted on that basis (i.e. - selling PageRank), due to the danger I referenced above.