It would not effect that much because Facebook offers "no follow" to links. About the content, it really depends what kind of store you have. If it is flashed based, Google crawler cannot read it but if it is not, to some extent, it may effect your SERP.
Would a facebook store selling all the same products as my website be classed as duplicate content by Google?
Generally speaking, no.
When it comes to duplicate content, Google tends to compare one entire web to another web page--so most likely your "pages" (as a whole) on Facebook are going to be different than what's on your web site.
Even so, if it possible to use different content and different descriptions on your Facebook page than what is on your web site it would be ideal to not have duplicate content.