The first thing to do is to identify the item’s flaw, or problem, or inform the buyer that the payment has not been received in full. Ask the latter to act to fix the problem, i.e., ask for the payment, a reimbursement or to send you the item. Negotiating amicably is always the cheapest and quickest solution (when it works, of course).
Practical tip 1: we recommend that you use methods of communication that leave a paper trail, such as email. If things escalate between the parties, you will be able to use the email exchange as evidence to prove your point.
Practical tip 2: it is possible that the Law protects you and that you could benefit from a reimbursement. Be careful, however, as in some circumstances, you may have a short deadline to ask for the compensation (sometimes as short as 7 days). We therefore recommend that you don’t waste any time and immediately notify the other party or merchant who sold you the good.