A scenario (fiction).
I see a build I want to try - it uses a Greene Sword. I have no idea what a Greene Sword is or what it's worth.
So I say "WTB Greene Sword"
Meanwhile some guy has a Greene Sword, he has no idea what is worth, but it looks cool so he's keeping it. But, if the price is right, he'll sell it.
So some guy pm's me and says "How much are you offering?"
Me: (I don't know what they're worth, so...) "How much do you want?"
Some guy: (he doesn't know what it's worth, but he doesn't want to sell a 100k item for 10k, so...) "How much are you offering?"
Me: (I don't want to buy a 10k item for 100k, so...) "How much do you want?"
etc, etc, rinse & repeat.
The point: neither of these people are trying to "rip off" the other, but simply trying to not be ripped off. Sure, if they were into buying and selling, and/or they knew where to look, they could do a price check before hand, but the bottom line is, it's just normal people doing normal stuff.
Or, at least, it
can be. There's no need to get your panties in a bunch over it.