I've never skimmed the peels off before... I'll need to try it. Didn't know that they affected the flavor. We generally make the tahini at home, store bought stuff is insanely priced around here. Would you recommend toasted tahini or non-toasted? I managed to track down some ground sumac at a local halal market. I'm a pretty big fan of it in hummus now.
Never heard of roaster/non-roasted, need to check what the normal one in Israel/Palestine is. The one I like the most is Al-Karawan and hell that is like 7 times the price at home O.O Sumac is an interesting spice to put in Hummus, never heard of anyone doing that but hey, it it tastes good :D We use it mainly in one special dish called Musakhan. It is basically tons of onions and chicken covered in massive amounts of sumac and served on bread soaked with the oil from the onions. One of my favorites and I just had a smell-memory activated by typing that! You should try it out :)
I think I started adding the sumac because of a random Lebanese food blog, but to be honest I'm not really sure. The Amazon question/answers section says that tahini is toasted. That pricing is pretty par for the course, unfortunately. Maybe a few dollars higher than normal, since it is a food product on Amazon, but not by much. :( I do love onions and bread...