Thieves and scammers abound so protect yourself.
When you get a receipt for an item ask for some
identification from the seller.
If you want to register any motor vehicle, you will need the VIN (number), hull number, serial number or whatever applies.
If you want to sell any item you can be asked for your bill of sale from the previous owner and the reverse is a good idea, as well.
Ask for the owner's bill of sale from the person he/she bought the item from to be sure that you aren't buying a stolen item.
Details of the condition of an item etc. that are only verbal between you and the seller are meaningless.
Put it on the bill of sale if you want the condition to be understood by both parties. Other wise it might be considered "As Is"