This is one of those "if you have to ask, you already know the answer" type questions.
If OP's conscience was totally clear, he wouldn't have made the thread.
If you feel like you deserve some sort of 'moral revenge' for being messed around with a long dispatch time, just tell them a convenient time for them to collect from you is 6pm on a Sunday or something.
Keeping items that don't belong to you isn't on though.
If I felt messed around by the seller I certainly wouldn't be going out of my way to do them any favours.
And for how long for? The point I'm making is, it will eventually get to a point where something needs to happen to the item whether you keep it, sell it, bin it, or send it back at a cost to yourself. If the seller is not interested in paying to return the item, then it's stuck with you, but as there hasn't actually been a sale, it's still being classed as theft.
Edit: I'm not agreeing with the OP's choices here, I'm just pointing out that "theft" isn't as black and white as people are making it out to be.
There are measures you can take. Recorded delivery, for one.
