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I've traveled in the past with a laptop in its bag as the sole carry on. There were usually other "small" electronic parts, like a mouse, charger, phone charger, etc.
The laptop and its accessories are the property of the company I work for and on whose behalf I'm going on the business trip. However, I have no official document that states the ownership of the device(s), such is the company policy (I can't change that, nor include it somewhere), but I do have a written travel order. Customs never made any issue about that, just as for a mobile phone, even in the case of two laptops or external hard drives. (I guess that that falls under some kind of "personal items" category.) They typically look through my stuff and just confirm that those are business things and don't look twice at them (once they asked me to power on the laptop, but that is for security reasons).
What is about to change is that I'm going to be required to travel, again on business, with a graphic card. Those are also company-issued and definitely not high-end (perhaps ca. 300 euro). However, these are graphic cards that are used for PCs normally, i.e. as such they can't be plugged in into a laptop. The interface for the laptop wouldn't look as such to the layman and can't be found on the internet, as it is internally built.
My main concern is that a customs officer will think that I'm trying to smuggle a graphic card and that I'm going to be either forced to pay taxes/duties or worse get penalized. I haven't been able to find any rules regarding electronic devices as to what is allowed that would match my situation - not even for just the laptop. The question is whether there are such rules and subsequently whether my concern is justified?
I tagged the question with europe, but to further narrow down, focus on entering and leaving the EU.
And to give a bit of substance to my concern, I've observed that non-technical people are often biased when it comes to electronic devices. For example, no one would accuse a man with a leather briefcase that he traveled to London without it, purchased there a 2000 euro briefcase and is now attempting to avoid paying due customs.
I don't think Am I allowed to take a graphics card in my carry-on luggage? is a duplicate for two reasons. First, it focuses on security rather than customs. Second, OP says explicitly that he is going to buy a graphic card abroad and consequently has all the required documentation and/or packaging and the duty and intent to pay any occurring taxes/duties, which is not my case. I have neither the packaging, nor the receipts, nor should I be obliged to pay duties and taxes.