This is not correct. A lot of people believe this but the law says otherwise. Almost all sales are governed by contract law (and all merchandise sales that occur on this site are definitely governed by contract law), and what contract law essentially says is that as soon as both parties agree to the same terms of the sale then there is a legally binding contract in place that both parties are legally obligated to have to fulfill as agreed, and even just a verbal agreement from the parties is usually sufficient for the contract. The buyer is then obligated to have to pay as agreed etc, and the seller is obligated to have to provide the item that was agreed on in the condition and for the price that was agreed to etc.
Now if the sale was conditional on certain things happening first, then if those things don't happen then the contract may no longer be valid. An example might be "I will buy the cue for x amount to be paid to you on Saturday IF I am able to get a loan from my father in law". In that case since it was conditional there may no longer be a contract if the buyer couldn't get the loan. But if they just said "I am agreeing to buy the cue for the x dollars that we agreed on and will paypal you by Saturday" and then went on to mention where they hoped to get the money in a loan from their father in law but did not make the sale conditional on it then a legal contract is almost certainly still in place.
But in any case where there are no conditional clauses as part of the agreement, then it is clear that as soon as both parties agree to the terms of the sale there a legally binding contract in place that both parties must fulfill. If the money is not yet in the seller's hands, or the cue is not yet in the buyer's hands, it does NOT mean that there is not yet a sale or contract or legal obligation. All it means is that one or both parties have not yet fulfilled all of their legal obligations for the sale under the contract that they are legally obligated to have to honor.
Aside from there being a legally binding obligation once both parties have agreed to a sale, I'd argue that there is also a moral and ethical obligation as well but fortunately we don't all have to agree on that since the legal obligation already clearly covers things.