According to the rules, it would appear that you are right and he was wrong.
From the thread on camp rules:
"Outdoor mobs on fast respawn such as HG and spectres, if you cannot engage immediately you do not hold the camp. Please try and share with fellow players in these instances." (
http://www.project1999.org/forums/showthread.php?t=2653)
As soon as he left to go sell he forfeited any claim to that camp. Since you had been there clearing repops after he left and were ready to engage the next pop it was legitimately yours.
Don't be afraid to stand your ground if you know that you are right. Try to /petition next time, as Skittlez suggested.