I just wanted to buy a tire

My wife and I were leaving town after delivering a mower to our son in law when a trailer tire lost part of it's tread.We had 100 miles to travel and it was 3pm. We drove to the nearest Sears tire center and asked to purchase a new tire. The attendant told us that they could not install a tire on a trailer. I asked to use a jack and lug wrench so as to remove the wheel for a "carry in" which he told us they could do. Even though the truck and trailer were in full view of the open doors,we were told that they would not allow us to use their equipment. What ever happened to common courtesy?