Here's my two cents: they are a small outfit. The same occurs in one of my favorite online stores, Worlds Records. These folk don't get huge volume discounts from distributors and they have higher overhead and they want to make a little money. You basically pay list price in both places. In the case of Mosaic, you get their filtering of all out there to what they feel is essential material. You pay for that knowledge and selection too. In the case of Worlds, they carry small independent labels that put out niche interest items and you can actually order then and receive them. You can get every Jazz Oracle from them for example; that is NOT the case with cduniverse, though you can sometimes get some of them from cduniverse for a bit less. You can also get from Worlds imports often a number of weeks before other distributors offer them for US release (Storyville, etc.) THAT is cool.
Now I don't usually buy from trubeblue because for me (as for many of us here) my interests are broader and I generally already have the items they offer of artists that intrigue me, or will preorder them elsewhere and save some dollars. I do support Worlds Records though with my business because I like their support of the small niche products, AND I can often only easily find the material in their stock. Great little outfit!