Is there any way to guarantee that an international european bank money transfer transaction (between private sellers) is safe? I mean, I will pay for a product, how do I guarantee that it arrives in perfect condition?
You can buy shipping insurance to safeguard issues with the delivery.
Shipping insurance is a service which may reimburse senders whose parcels are lost, stolen, and/or damaged in transit. - wikipedia
If you're worried about the transaction itself (e.g. not getting what you thought you bought), there's internet escrow.
As with traditional escrow, Internet escrow works by placing money in the control of an independent and licensed third party in order to protect both buyer and seller in a transaction. When both parties verify the transaction has been completed per terms set, the money is released. If at any point there is a dispute between the parties in the transaction, the process moves along to dispute resolution. The outcome of the dispute resolution process will decide what happens to money in escrow. - wikipedia
Note that internet escrow is conceptually independent of the remittance mechanism (IBAN, paypal, credit card, etc). If you want to use IBAN specifically, you will need to find an internet escrow company that can receive money that way. In that case, your IBAN payment would be made to the escrow company, not to the seller directly. The escrow company is responsible for making the payment to the seller later, once everything has been found to be as agreed.