Manufacturing, qualification and sale

If it's a consumer gadget/toy (and not a medical device which has much more stringent standards!) you probably just need to worry about safety standards and EMC standards.

• CE declaration for Europe
• EMC certification: in the USA, FCC class B radiated emissions
• Europe now requires RoHS to limit Cd, Pb, Hg and some other hazardous substances

Safety standards will include things like pinch points, sharp edges; if it's a toy and there are children involved, there's a standard container used for choke hazards, etc. If the gadget plugs into the wall for power, you need to worry about shock hazards and fire hazards (UL in the USA), although you can probably deal with this by using a UL/CE approved wall transformer so voltages entering your device are limited.

If it's a small batch, and you're using an approved wall transformer as mentioned by Jason S, you probably don't have to worry about anything. I've seen all sorts of sites selling little gadgets they make in small quantity, that I am 99% sure they didn't do any kind of certification. It all depends on the scale...does 'small batch' mean 20 or 2000?