CESSNA PLT DEPARTING AN UNCTLED FIELD AT NIGHT TAKES OFF DESPITE HEARING INBOUND ACFT ADVISING ON SHORT FINAL FOR SAME RWY.

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CMA RWY 26 (6010 FT). TWR CLOSED. IFR CLRNC FROM APCH (BY RADIO ON THE GND). APCH ASKED IF I WAS GOING TO BE READY SOON FOR TKOF. I HAD PREVIOUSLY COPIED CLRNC AND HAD TAXIED UP TO HOLD LINE AND WAS COMPLETING RUN-UP. THE ARPT WAS DESERTED AND I WAS POSITIONED FOR A GOOD VIEW OF INCOMING TFC; NOT JUST PERPENDICULAR; BUT 30 DEGS TO THE R TO SEE APCH BETTER. I RESPONDED TO APCH THAT I WAS 2 MINS AWAY FROM BEING READY. IN LESS THAN 2 MINS; I TOLD APCH I WAS READY. IMMEDIATELY; APCH CAME BACK TO ME AND CLRED ME FOR TKOF; TOLD ME TO SWITCH TO CTAF FOR APPROPRIATE COMS AND THEN COME BACK TO APCH FREQ WHEN AIRBORNE. I SWITCHED TO CTAF WHILE LOOKING FOR INCOMING ACFT. I HAD BEEN WATCHING EVEN BEFORE THE FOLLOWING RADIO CALL WHICH I HEARD THE END OF; '...ACFT Y ON 1/2 MI FINAL.' I LOOKED CAREFULLY AND DID NOT SEE ANYONE -- WHICH I RECALL IS ODD SINCE LNDG LIGHTS SHOULD BE EASILY SEEN; ESPECIALLY AT 1/2 MI (THERE IS A DISPLACED THRESHOLD THAT IS QUITE LONG; EASY TO SEE THE LINE-UP). BASED UPON MANY EXPERIENCES OF PLTS MUCH FURTHER THAN WHAT THEY SAY AND THINKING APCH WAS RUSHING A BIT BECAUSE OF TFC THAT HE KNEW WAS UP THE LINE; I ANNOUNCED INTENTIONS: 'TAKING RWY 26 FOR DEP TO THE W' AND ACTED ON THIS IMMEDIATELY. WHEN I WAS AT ABOUT 150 FT (NOT AT THE END OF THE RWY YET; 6010 FT); I LOOKED BACK AND SAW HIM ROLLING OUT (PERHAPS 20-50 YARDS AFTER TOUCHDOWN; JUST PAST THE NUMBERS). HE SAID TO ME ON CTAF; 'THAT WASN'T A VERY SMART MOVE.' (HE'S NOT HOSTILE -- HE'S RIGHT!) OBVIOUSLY; THIS IS MY ERROR. I RUSHED AWAY FROM THE ATC RELEASE CLRNC (MAKING ASSUMPTIONS). I FAILED TO WORK THE RADIO AND MADE 2-WAY CONTACT WITH THE CITATION AS TO MY INTENTIONS AND TIMING. I ASSUMED I COULD SEE INCOMING TFC THAT MIGHT REALLY BE CLOSE -- ESPECIALLY RELYING ON LNDG LIGHTS BEING VERY VISIBLE AT NIGHT. I WAS AWAKE; HAD JUST REFUELED FROM A 1 HR TRIP; AND IF YOU CAN BELIEVE THIS -- NOT IN A HURRY. IT SEEMED SAFE NOT HAVING SEEN THE INCOMING ACFT. THE ONE THING THAT WILL CHANGE FOR ME IS SLOWER TRANSITIONS WHEN USING 2 FREQS -- ATC FOR THE CLRNC; AND BACK TO CTAF FOR THE DEP. I HAVE LOTS OF EXPERIENCE DOING THIS ON APCH; BUT VERY LITTLE DOING IT FOR TKOF. ALSO; AND PERHAPS MOST IMPORTANT IS THAT I FAILED TO HAVE DIRECT COM WITH THE INCOMING TFC IN ORDER FOR US TO COORD OUR OWN DEP AND LNDG.