If an item is priced to sell you will not have to deal with low-ballers because it will sell before they get a chance to even see your ad. That is why explicitly saying "Price is firm" is unnecessary. It just makes you look stubborn and will scare away potential buyers.Well, maybe. It sounds like it will simply scare YOU away.
Look, I don't often list gear as "firm" but if/when I do it's because there are a lot of low-ballers out there.
I understand that your job is to get gear as cheaply as you can, just as mine if to get as much for my gear as I can. There's usually a happy compromise that makes for a win-win. But your first post makes it sound (to me) as if you feel that sellers owe you something. Other than a complete description and and timely communication, what else is a seller obligated to provide?