You're not too clear.
1) Someone is selling a domain and you post "SOLD" in the thread but someone else PM'ed the seller with a higher offer? No big deal - it's business - 1.
2) Someone is selling a domain as a fixed price / BIN and you postr "SOLD" but someone else PM'ed the seller with a higher offer? An annoyance but show me a seller who wouldn't sell a domain higher than their BIN - 3
3) You posted "Hey, I might buy this domain, what do you think?" and someone else contacts the owner to buy it for more? Again, an annoyance but still - business is business. - 2