The tills are hidden in product displays located close to the Genius bar. As for cash, I recently saw someone count out over $ 8,000 in $100 bills and the Apple person didn't bat an eye.
I've purchased Macbooks with cash and it's never been an issue. I tried to buy a Macbook with my Credit Card once, and the CC people declined the sale and locked my card as stolen. It took a bit of arguing with the CC people to get my CC unlocked. The CC people said it was "out of your pattern for purchases". In the meantime I simply went to the bank and got cash and paid cash.
I found out that you should call the CC people in advance and they will hopefully make a note of it, but they did tell me that even if you call and they make a note the transaction can still be declined. Since then every time I buy things from Apple store I carry cash just in case. The CC people hate me - I pay them every month in full, so perhaps that's their way of getting back at me. Apparently MacBooks and other expensive products at the Apple stores (and perhaps BestBuy) are prime targets for people trying to make quick purchases with stolen CC.