I have been doing some experiments trying to figure out how products are ranked in Google Product search.
Using computer memory as an example
If I make a general search on memory, results listed by relevence simply list products by supplier, how the top supplier is ranked does not seem to make any sense whatsoever. Its not because the have checkout, its not because they have adwords or good ratings, or even the best price. It seems completely random. But the same company is consistently there.
On a specific part number the results are again pretty random when sorted by relevence. None of the above seems to stand out as a possible source. Keyword density also seems to have little effect.
These results are definately not random as the same suppliers are consistently at the top. The only thing that I can see may be a factor is the expiry date of the products or extra information added in some of the category fields.