Quoted from boneman91:To me, the word FIRM means "take it or leave it" and has an standoff-ish, uncompromising attitude attached to it.
I agree with this. It's usually a somewhat unmotivated seller. More interested in the money than moving the item.
I never put FIRM or OBO in an ad, and I still receive both full price and below asking.
Some people will make offers, some people will pay full price - you should just expect it if you're selling a used item. The amount of BS you deal with depends on the the price you set.
Although it may reduce a potential buyer's interest, saying "Firm" doesn't help sell an item.