Lesson learned on resellers - buy from Apple next time.
You don't have to. It's not about Apple vs everybody else. There are resellers that might offer even more service than Apple for less. You don't necessarily have to buy Apple products directly from Apple. Suggesting it is a bit stupid. There are consumer rights, you know.
Even if you want AppleCare, buy the item from a reliable and trustworthy reseller and buy the AppleCare separately for a fraction of the price that Apple charges. There are several discussions about AppleCare that is not being sold by Apple Inc. It's just common sense and you have to know that AppleCare is linked to the computer - if you buy one that had been registered with another one, that might not be worth the disc and paper that it's printed on.
Generally speaking, if you buy something online or on the phone, you have the right to cancel within a certain period of time and send the item back as you received it.
IF the reseller made the mistake of sending something that is not what you ordered, even the shipping cost should be covered by them. I didn't even know that these iMacs shipped with the wired mouse and keyboards, but unless they clearly stated that yours would include these; you should have no problem of getting a full refund or having a new set of wireless mouse/keyboards sent to you upon you sending back the wired ones.
Anyway, don't use the computer and call the reseller - this has nothing to do with Apple. If you purchased the computer from Apple, the same legislation would apply. Don't let them charge you with a stocking fee and there is no resolution, ask PROFESSIONAL consumer advice. I am based in the UK, here we have Consumerdirect that provides free and impartial consumer advice.