And to keep the car comparison going: during the summer, bike shops routinely book two weeks out, which is almost unheard of with car service.
I'm both surprised and not surprised to hear that clothing is not a big seller--I've never been a fan of getting clothing without trying it on. However, once I know how a brand fits, that's not as much of an issue. It is surprising to walk into a bike shop and want some shorts only to find they don't stock anything under $80--you really have to work hard to find $80 shorts at the mall. So maybe the problem is they need to do better at picking what their customers want.
Unfortunately, I don't have good ideas at what they should sell in addition to labor. Need it now items are great (tires) but it's hard to make all of your money just on mechanics.