A good clue to when a company has opened up a "third party seller" thing is when you look on their website and it either doesn't give you info about which stores stock the item, or they have an incredibly huge choice of items which would not be possible to fit in a normal sized (or even large retail park) sized shop. That's when you want to look for those little "fulfilled by XYZ" things.
For example, on that website, The Range apparently have a selection of over 1,100 bookcases. The Range is not even a furniture specialist - e.g. Ikea (who specialise in furniture and have gigantic stores) have 357 bookcases (and Ikea list every single colour/variation of an item as a separate product).
They don't let you filter out the "R Marketplace" products but I bet if you filtered those out you would be left with less than 100. Argos, which is closer to The Range than Ikea, and hasn't so far done the 3rd party thing, has 95 bookcases on their website.