If ratings are high, it's a safe guess that more viewers are watching thus more toys will sell
Not... really. In fact, as some have mentioned there are quite a few examples where the two seem to go in polar opposite directions. Hibiki had some of the highest ratings of Heisei Kamen Rider and the lowest merch sales; Den-O had very high merch sales and pretty dull ratings.
I mentioned in another thread, but Ohranger's toy sales actually went up (it became the highest-selling sentai of the period) even as its ratings went down following the sarin gas attack tone shift.
Obviously we'd need a much larger data set and maybe some charts to see if these incidents were outliers and one could pull any more general correlation between the two. But I think the connection is probably tenuous.
As we've seen in this thread alone, ratings can be influenced by a number of factors, and these factors are likely completely independent to what sells merch. I think this is just common sense; while many of us will buy merch based on what shows we like best, our decisions are also often determined on other factors as well, such as the quality of the merch itself. Certainly we've seen no lack of people just on these boards raving about toys for shows that they otherwise couldn't care less about. Similarly, I'm not going to waste my money on crap toys no matter how much I love the show built around them. (ex: GoseiGreat I would buy. GoseiUltimate I would HELL OF buy. Mystic Runner, hell no.)
I think it can be meaningful to analyze ratings, but you need to do it in a broader context rather than just looking at the straight-up numbers; this is what the producers are doing too, after all. If Goseiger's (and SHT's) ratings were sinking completely independently of any other factors, it probably
would be a bad sign for the block. But knowing that the falling ratings started at exactly the same time a Pokemon variety show at the start of a new generation began airing in the timeslot muddies the picture substantially.