Well, if you have a very valuable item, you will receive more money for it if you sell it to an invested buyer. Example: you have a sword worth 2,000 septims, but the richest merchant in the game has 1,500 septims. If you invest, you'll get 2,000 back instead of the 1,500 you would have gotten if you hadn't. Of course, you won't get the exact value of the item, but that was just an example.