I just follow the principle that if you can't say anything nice, don't say anything at all. Saying "Wow, that's a nice coin, wish I had more spare cash" gives the thread a free bump (so the seller doesn't have to look desperate bumping it themselves) and also lets others know that there is interest in the item (so it doesn't look like a rotting fish that nobody wants).
The problem with sending an offer is if you do not have an answer till next day and the price is dropped 5% overnight.
I agree with Big A.D. that Positive comments are often useful... It's the Negative ones that some Very Rure Specimen use that are the problem. In reality, those Neg. comments have nothing to do with fairness. If you don't like the price, for whatever reason, shut up & look away ! Even worse, to argue openly with the Administrator is totally 'out of order'. Basic courtesy would dictate a Private message to clarify an issue. If you don't like the Rules of this Forum, no-one's forcing you to stay... even better, why don't you create your own Forum and see how it goes !