rsvp
It's nothing to do with your lack of knowledge about the printer. The same thing could happen to anyone.
Have a look at your receipt. Does it have the name of the salesperson on it?
If so, contact the store, ask to speak to the manager, and tell him/her the whole story.
Tell them that you are not prepared to return to the store because it will cost £X.XX. Suggest that possible actions will be to send the part back through the post (and you should add the cost of postage and packaging) for a refund by cheque or directly to your bank account. Ensure the part is sent back recorded delivery by the way, or they might say they didn't receive it.
If they are reluctant to do that, tell them that you are happy for them to come up with an alternative solution, and get them to agree a time to DSL, you back.
If this doesn't work (or they don't call you back), call them back and ask for the telephone number of customer services, and for the manager's full name and the name of the sales person.
Don't tell them what you're going to do. Just ask for the information.