I was just busting his balls.
Typically I would (as a buyer) wait until the seller responded to make another offer. It automatically renders the first offer moot if you offer more.
Say the seller replied to my less-than-asking-price offer and did not accept it. Without the full price offer below mine, I'd have immedately upped my offer to the full price...which is what I was really willing to pay in the first place. I just offered $5 less in an effort to save $5.
No rule against it or anything- just not the most gentlemanly way to handle it.