How I handle situations like that really depends on how the customer does. In general, if a buyer contacts me after purchase and lets me know they aren't going ahead with the transaction, I'll accept any reasonable explanation and offer to send through a mutual cancellation. (Reasonable explanation for me includes buyer error, like admitting they didn't read the listing in full, then realised it wasn't going to be suitable, or - as has happened a couple of times recently - buying after receiving a voucher code that didn't work when they went to pay).
I do mutual cancellations in those circumstances because it's quicker and easier all-round (I just feel it's better for customer relations, sometimes (like in the case of the vouchers) it's not the buyer's fault, and I can put the item back up for sale straight away), plus they've indicated they'd be amenable to accepting a cancellation and at least the buyers contacted me very quickly and let me know, rather than just bought and were never heard from again.
If I had to contact them (which I don't do until around day 5 or 6), and/or I got a message that didn't offer a reasonable explanation, I would most likely do the UPI dispute (theoretically, anyway - that scenario hasn't actually happend yet 
So far, every time I have to follow up on an unpaid item, the buyer either never responds at all, or pays).
