I've looked into it and, while I've never pulled the trigger, I would guess that it would likely increase sales to a degree.
My question has always been more along the lines of "how" I would be able to market it against the "big guys" who spend tens of thousands of dollars every year (maybe more) driving people to their design sites with online, print, and media placements.
My niche has always been local, small groups that the screenprinter shops either aren't interested in or are priced too high for small numbers. I also do the Etsy thing that produces an average of about 10 orders per month (small online store).
I'm kind of anxious too for any feedback on the success of the online designer ......