Wasn't having a peak pretty much the only reason he joined the mining team? He certainly didn't do it for the pittance the miners earn. He presumes he needs the power to clear an A rank dungeon to clear the rest of the special demon dungeon. However, as long as he's not officially an S rank hunter, he has no way to get the rights to an A rank dungeon. Unless it's by accident, like a lower rank dungeon turning into a double dungeon (which is what killed him back then) or a sudden red gate that happens to be A rank in reality. But he can hardly count on those possibilities.

Selling the artifact would be stupid indeed. The stuff he can buy from the free market is of much lower quality, even if it was fitting for him. We can assume the big guilds keep the good stuff for themselves. I suppose he's feeling so much pressure time wise that he even considers selling it.

But yes, maybe he was a bit too eager to have a peak if he allowed someone to sneak behind him. He must have got too comfortable with the idea the dungeon will be absolutely empty when the lunch break happens.