House speakers in the main are 8ohm (as yours are).
Car speakers and amps are generally anything from 1.2 to 4 ohm depending on manufaturer (4 ohm used to be the more common)
What issues would this cause?
Well the resistance of the house house speakers is higher, so it would need more power from the amp to get the same volume, which would put more stress on the amp.
Depending on the quality of the amp and tolerances it may cause the amp to fail.
You would need to check the specs of any amp you plan to use to see if it can handle 8ohms.
I'm sure soundman will along soon to clarify more. (and probably disbunk the above)