I see.
By the way, eBay nowadays is much easier than that. When you list something, eBay automatically calculates the shipping fees based on dimensions, weight, and bidder's address (and shows it to prospective bidders while the auction is active, so they don't bother you with that). You can offer regular mail, UPS, and/or FedEx. If you've kept the iMac box you can just use it as a parcel, otherwise buy a large parcel and foam peanuts or bubble paper at your post office or Staples for about $10. You can ask the carrier to come pick it up for a couple of bucks (sometimes free), or bring it yourself if you prefer.
You can also mention in your auction that this or that isn't included with the item you're selling, and there are actually many buyers who already have their own keyboard, mouse, etc. so they're not interested in getting yours.