If someone comes to your sale and wants to buy the item, but cannot move it then, they MUST pay for the item before they leave. Not leave a deposit. If they have to go get cash, give them 30 minutes. After that if someone wants it, and has cash, sell it to them. If they give you a deposit of $30 or whatever, write out a contract in pen that the $30 is to hold the bed for thirty minutes/1hour so that they can go get cash and return to pay for the item. After the time period is up the item will again be available for sale and the deposit will be returned to the other party as long as he returns before 7 pm on whatever the date is. Have ALL of that written into the contract (write out a couple and keep them on hand before the sale, to have the blanks filled in when needed). Insist that both of you sign it, in pen, and if possible have someone you know witness it. It protects you both. YOU keep the contract with the deposit.
Otherwise you might get a deposit and hold the item when other people might be interested in it. By the time that guy flakes out and you know it, the sale would be over. IF someone pays and doesn't come pick it up, make sure they KNOW that you will be leaving the furniture outside when you go to bed. If it is stolen it is THEIR problem for not picking it up in a timely manner.
IF someone really wants it, and it is being held for someone, by all means take their names. Let them know the time that the hold expires and call them promptly if the first deal falls through.