I agree. My explanation is that numbers like "$1,000" and "$999" seem like opening bids--so not only are they arbitrary, they're higher than what the seller is

I agree. My explanation is that numbers like "$1,000" and "$999" seem like opening bids--so not only are they arbitrary, they're higher than what the seller is willing to accept. Whereas "$1,012" at least seems like it's already done some negotiating with market forces/demand.