If you sell an option at market, you can actually receive less than $.05. I sold some APPX Dec 40 calls this morning and got $.02 for them. Spread at the time was .00x.10. Better than nothing, I guess, but I was surprised the minimum wasn't .05.
For offsetting transactions, that may be true. For initiating transactions, it may not be true. Contact your broker to be exactly sure.
Your order was most likely routed to Boston Options Exchange (BOX), which offers price improvement (PIP) for orders submitted at market quote. Visit BOX website and read up on PIP.