OK, lets see the opposite situation. You are the buyer of domain listed at Sedo. You noticed one domain with BIN $500 and purchase it. The seller decide not to transfer it as he thinks he could get more (the opposite of this case where buyer thinks he could get it for less). Would that be OK ??
Recently I had situation where I forgot to update BIN and sold great domain way under the amount I wanted as someone hit BIN. Transferring to the buyer was not a question at all. He honestly paid and I transferred it immediately, although I was pissed off.
If you make an offer, you are supposed to pay if accepted. Period. It would be something else if your offer would be accepted few weeks later, but if it was accepted right away you should pay.
You cant know what loss could cause your offer to the seller if you don't pay. One more example, I was in negotiation with one Chinese regarding valuable numeric domain. He offered high $xxx and I accepted. He did not respond on my email 24 hours and in the meantime I received direct e-mail inquiry where buyer from US offered to pay the price listed on GoDaddy less fee. It would be better deal for me, and much easier (you know why if you ever dealt with Chinese who don't speak English), but I answered him that I sold the domain and could let him know only if buyer fail to pay. The next day Chinese guy made payment.
I could get few more dollars, but I feel good, sleep well, keep my moral, AND I now am connected with solid Chinese buyer for future deals.