If you want to market small circuit boards from the EU into the EU, you need to register in all EU countries. It's not 100% clear whether the requirement is incurred at "making available for sale in country X" (ie as soon as you put it on the website) or at the point you make your first sale.
This is on top of trying to untangle what CE marking testing may be required.
On the other hand, if you're a Chinese small board vendor, you just advertise it on AliExpress, put it in the (subsidized!) small parcel post, and ignore the EU rules.
If you want to market small circuit boards from the EU into the EU, you need to register in all EU countries. It's not 100% clear whether the requirement is incurred at "making available for sale in country X" (ie as soon as you put it on the website) or at the point you make your first sale.
This is on top of trying to untangle what CE marking testing may be required.
On the other hand, if you're a Chinese small board vendor, you just advertise it on AliExpress, put it in the (subsidized!) small parcel post, and ignore the EU rules.