@RedCardinal, I agree. Throttling by using a conversion metric seems extremely unlikely to me, too. I think that it more likely to be just some kind of a coincidence that occurs at lower volumes of sales.
At the same time, conversion optimization is something every e-commerce site should be doing. If you can increase revenue by 20% from the same traffic and ranking levels - that's awesome. Then if traffic goes DOWN while the conversion percentage increases, you have a better reason for suspicions. I have never seen anyone with that situation so far.