Give every sale one record
A sale that exists only as a comment or a reply is hard to find later. Each sale needs one record with the buyer's name, what they bought, the price, the date and time, and where it is going. One row per order in a spreadsheet works at first. The point is that you never scroll through a conversation to find out what was agreed.
Settle the last-item question with a rule
In live sales, two people often claim the same item within seconds. Buyers accept the outcome when the rule is stated beforehand and applied the same way every time: first to order wins, or first to pay wins. Say it at the start of the sale. If the rule is first to order, the time on each order record is easier to read than a busy thread.
Track payment as its own status
Unpaid orders hold stock that someone else may want. Treat payment as a status separate from the order, for example new, waiting for payment, paid and shipped. Set a time limit for payment and tell buyers what it is.
Mark an order as paid only after you see the money in your own account. A screenshot shows what the buyer's screen displayed, not what you received.
Collect the details up front
Missing details cause avoidable problems: a phone number that does not work, an unclear address, a size nobody confirmed. Ask for the delivery details and any choice, such as size or color, when the buyer orders, not in a follow-up message.
Let buyers check for themselves
Many messages asking where an order is come from buyers who have no other way to find out. A link they can open to see the current status, and the courier and tracking number once you ship, saves you the reply and gets the buyer an answer sooner.