There is a difference in an initial offer and a "counter offer". The initial offer, (from the buyer) does NOT lock the sale out to other offers or Buy It Now.
If the seller does a "counter offer" it does indeed lock the sale down for 48 hours or until the buyer agrees to the counter offer or declines the counter offer or times runs out, (48 hours).
If the seller does a "counter offer" it does indeed lock the sale down for 48 hours or until the buyer agrees to the counter offer or declines the counter offer or times runs out, (48 hours).