I don't really think you care about the answer, but here is the comparison anyhow from a seller that uses as many platforms as I can:
Poshmark: 20% service fee, flat rate 5 lb max shipping. Prices inflated to cover 20%. Mostly fashion items. Focused now on "new, boutique sellers" aka drop shippers from wish, gilt, and online retailers. Letgo: local only. No fees, no option to ship. Buyers almost nonexistent in past few months. Used and new random items mostly cross posted on ou. OU: same random new and used items. Free local. 10% service fee option to ship. Flat rate shipping tiers usps Mix of lowball offers and serious buyers. Mostly the former.
I have an inconsistent stream of buyers all at once, then none at all on ou and poshmark. Its all free to list, so wherever it sells, it sells. Usually hope it sells for a 10% fee instead of posh's 20%, but I take all the sales I can get.