Take a look at this :
These two articles will tell you slightly different things, I let you judge which one convince you the most. But here is my approach :
If your product is called the Waise Watch, and you're only selling your Waise Watch on that site, go for singular. WaiseWatch.com will definitely call the user's attention on THE one and only Waise Watch. If you are, for example a watch retailer, and you sell many different watches, then go for the plural name. The impression will be like "the watches sold by Waise" or something like that.
Anyway, the singular will emphasize the fact that your product is unique, plural will emphasize the fact that you're selling a wide range of things.
But obviously, the best option if you want to have maximum visibility would be to buy the two.
Edit : look at this regarding domain name matching branding name http://www.bloomberg.com/news/articles/2012-05-11/when-should-domain-names-match-company-names , there is a lot of pertinent information.