I'm new to the forum and relatively new to poshmark--I've had both good and bad experiences as a buyer and seller. For the most part I take the stance that something is only "worth" what someone else is willing to pay for it (not what I paid or however much I lose to fees) so I don't get too bent out of shape over ridiculous offers... I just say "no" and forget about it since it's just a way for me to reduce the size of my shoe hoard to make room for other purchases.
It's much much better now, in my opinion, that they have the offer option to negotiate without it remaining in comments and crushing your sales with other potential buyers who want to further drop the price based on the last offer when the first (or second, third etc) person disappears. I don't hold items--and if I see something I want that is being held for someone else, I definitely swoop in with a better offer or the same offer with immediate payment! Maybe I'm too cut-throat... I just figure that if there is an expectation that a sale is reserved then there's really no reason to complete the transaction sooner rather than later and that just hurts everyone in the long run.
I do get annoyed on behalf of the seller when I see people asking for trades on a designer item in good condition! I swear their closets are always worn out flip flops, overused Victoria's secret pink yoga pants, and used beauty products past their expirations! Pretttttty sure the seller isn't going to trade lol.
As a buyer I get pretty annoyed with the excuses and sob stories--some of those sellers apparently move every other week, have a family death every weekend, and get Ebola/have a baby/have a terrible car accident on the days ending in "y" explaining why they haven't been able to get around to shipping anyone's purchases within a few weeks of sale for the last several months of people attempting to make purchase (because a few days is just an unreasonable expectation given the perfect storm of excuses they've concocted haha)
So far I haven't been burned by any fakes but I've mostly just tested the water with some smaller purchases from sellers with really good closets. Hopefully that's an indication that they are good sellers with authentic items in good condition and I won't be disappointed. All of the things with which I've been disappointed definitely had warning signs. I haven't tried to sell any really high-priced items because the fees are pretty high. I'm kind of unenthusiastic about the Vegas post offices to try ebay-selling and posh does make it reallllllly easy to ship. Mixed bag for me but I'm still optimistic!