..............Anybody know if running a 100 in the place of a 110 will cause any problems? 
Just as Chris Liston said earlier, it might (actually probably will) drop into turns a little quicker ..... but I'm confused. Why do you prefer the 110 size?
What bike are you running this tire on? Wheel type and size?
I know that the 100/90-19 is probably the most common replacement for the OEM 3.25-19 size (most SOHC4 models) and you ordered one size larger than that. I looked at the dimensions of the metric sized tires (compared to stock tire sizes) and chose the 90/90-19 front with a 120/90-18 rear for my '75 CB750F. The front is as close dimensionally as I could find, the rear is definitely larger than stock. Handles fine. A good friend who rode SOHC4's (500's and 750's) in the 70's and 80's (also rides an '83 CB1100 DOHC4) just got a CB750F and he liked the way mine handled so much that he bought exactly the same tires.
One point which would concern me is that the wheels on these bikes are rather narrow, most likely smaller than the smallest acceptable width to the tire manufacturers for the 110/90 section tire. This tends to squeeze the beads together and gives a different profile to the tire than the designers intended it to have. For that reason I personally would not use that size (on a stock wheel that is). I always check the wheel width when switching from older inch sizing to metric tires and try to match the wheel and tire sizes.
Now I TOTALLY agree the the merchant is acting like an A$$HOLE and no one should have to deal with that level of "customer service" (and I use that terminology very loosely), but depending on your intended use, it may turn out to be an error which is a better selection for you.
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