Generally speaking ...
Singular = Brand
Plural = Keyword intended for sales of the specific product
Unicorn = Nice brand name for just about any company
Unicorns = eCommerce site selling Unicorns
Keep in mind there are pretty much infinite exceptions going both ways.
As others have pointed out .. sometimes the plural doesn't even have an S at the end.
I don't think an S at the end of Seafoods is necessarily unworkable .. I've seen S at the end of food (Whole Foods I think is a big brand) .. I think foods likely falls into that exception .. but I would still take the advice above in that I think seafoods with an S might be a little less usable than without compared to just foods. But it definitely depends on the other word.
Also others have also mentioned, just because a name is listed for sale at a certain price does not mean it's actually worth that price at all.
Also .. a general % difference between singular and plural doesn't really exist .. it's really case by case. But these days the "brand" version is usually what people go for. But if the product is big and has a lot of sales online, then getting the plural for eCommerce purposes isn't a bad thing .. but for me in such cases the search numbers need to be very high and the domain needs to be very specific .. otherwise the domain is just about worthless more often than not.
Good luck with it!
