I accidentally bought an iMac on eBay. I was planning on buying this later, I messaged the seller that I accidentally bought it an I could pay it in 1 to 2 hours. Can I do that? Or is it *immediate* payment?

I'm an eBay seller and it really depends on the seller. Some sellers give a few days so you can pay and others want an immediate payment. But , usually they give a couple of days.

I don't know how you "accidentally buy something". If you did it then that's up to the seller. If you back out of the deal and refuse to pay you could be slapped with a strike by Ebay.