Receiving multiple international wires and declaring lower values

The very fact that you are posting this indicates that you know the answer. Here are some other ways to consider your path...

Mirror Test. Imagine making the decision and then look at yourself in the mirror. How do you feel? What do you see in your eyes? Does it trigger alarm bells, violate your principles, or summon a guilty conscience?
Front Page Test. Imagine reading about the decision in the headlines, with your name and photo prominently displayed. How would you look and feel, and what effect would it have on your reputation, relationships, and business prospects?
Consequences Test. The consequences of unethical decisions can be severe, so it is wise to consider them in advance. Think not only of the potential monetary costs associated with certain causes of action but also the reputational costs (brand equity), relationship costs, and psychological costs (the burden of regret).
Reciprocity Test. Imagine being on the receiving end of your possible action: how would you feel if the shoe were on the other foot (and you were getting kicked)? Would you be violating the golden rule?
Universality Test. Drawing on the thinking of the great German philosopher Immanuel Kant, imagine if your course of action were to become standard practice for all people in all times (a universal maxim). Would that be a good and just outcome?
Role Model Test. Given the choice before you, what would your role model (or someone you admire, such as a parent, sibling, friend, mentor, or coach) say or do? Have you asked him or her?
Loved One Test. This can work in two ways: First, similar to the role model test above, what would your mother or father—or grandmother or grandfather—say or do about this? Second, imagine your son or daughter (or other precious loved one, such as a spouse, close relative, or best friend) in this situation and think through what you would want to have happen.
Advisory Group Test. Do you have an advisory group, a men’s or women’s group, a “personal board of directors,” or other group of people you can run this by to gather input and explore options? If not, why not start one? Or reach out to people you trust in this instance. Often, they have valuable experience or insights.
Sanctuary Test. Finally, have you given it time for reflection? Have you slept on it? Have you prayed about it, or meditated, or otherwise found sanctuary to let your inner voice emerge?

I've never heard of the authorities examining bank transactions to verify shipment values - certainly not for individuals - but then maybe it's different in Europe?

In the USA that is considered Federal wire fraud and bank fraud. But don't worry they will probably just send you to one of those nicer non-violent offender camps in a pleasant climate.

We all make decision to short cut.
But I would want it to be my decision and not made an accomplice.
That is why I always state that I will declare the full value. I do not like to lie, even so I can emphasize with the cause.

If I am not mistaken (which I may be), the buyer slash importer is responsible for the customs declaration. It is counterintuitive because the seller / shipper completes the form.

It's up to a importer / exporter to know all state and federal laws prior to import/export. Look at any countries custom page

Don't forget all, once a government form is filled out by the federal Government at a border it outlast you.

( for above scenario, if just say it was picked up that there was deception on the watch decleration and a form was filled out with buyer and sellers name and addresses. 10 years down the track you want to import export to the said country and need to apply for a permit to do so. Don't you think being in the system for false declarations on goods would help or hinder such application. )

So law is risk based now. 😒 Would like to hear your rating for some other laws as deceit and deception is higher in my book.

We are not just talking about customs declarations here. Moving money in a manner to deceive the Gov't is called money laundering. You don't want to go there.

Why are you even entertaining this? Don't be silly. Either find another buyer or tell him that you won't conspire to commit fraud.

There is also the real possibility it is a scam.

Well in my country the government wants 21% + additional costs of everything that comes into the country from outside of the EU be it brand new ( which I can understand ) or be it 2nd hand. They make no distinction between the two.

For 2nd hand goods I totally disagree as the tax has already been paid somewhere at some point and it has got nothing to do with the government here. Legalized theft IMHO.