Hi, i am developing a project wich involves some "shields" available in ebay, the project combines several things, like for example a bluetooth interface "shield" and a mp3 player "shield" i wanted to know if it is legal to sell a product that uses those shields, i think it should be because i bought the shields, i make the code and verything else...
I don't understand the question. What you really want to know? If you can buy and then resell?
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Generally what you describe is legal. Google "can I embedd an arduino in my commercial product." Many of the shields have even less complicated legal rules. The exception would be when you use boards that are clearly falsified knockoffs of adafruit/sparkfun etc board (meaning they are NOT from those manufactorers but they say things like "adafruit/sparkfun".) You really should not be packaging those in a commercial product.
Generally though those companies are especially awesome and they would never target anyone for something like that. Plus if you ever got to the scale where you were a target, it would be pretty silly if you were still using shields.
Remember the shields have low QC and you want to account for that in your design.
Rggmiranda:
Hi, i am developing a project wich involves some "shields" available in ebay, the project combines several things, like for example a bluetooth interface "shield" and a mp3 player "shield" i wanted to know if it is legal to sell a product that uses those shields, i think it should be because i bought the shields, i make the code and verything else...
Hi nice to meet to and also welcome to the forum.
nah just sell what you want, anyway no factory intrest in a tiny seller 
There is no problem selling on your gadget for experimentation.
If it sale for profit or product use you may fall foul of CE or Federal emission regs.
AFAIK most arduino boards are not tested for this.
But in any case its the complete assembly that has to be tested whatever the testing done on the parts themselves.
There are exemptions for kits and parts for experimental use.
The testing required is unfortunately very expensive , best case of pass first time would cost 10K+.
Testing of arduino components individually I have done indicates nearly all would fail.