my personal experience with online selling platforms, the good, the bad and the ugly...

1. eBay
The pros for selling on this giant leader are common sense: higher traffic, support for international markets, OK customer service and buyers/sellers protection.  

The cons:  
- Too much competition that your listings hardly get seen unless you're willing to pay extra to promote.  
- The listing form is too long and complicated. 
- Fees! eBay's so called "free insertion fee", usually limited to ~50 listings a month, is a scam deal considering every other site offers completely unlimited free listings.  "Good 'till Cancelled" listing now auto-renew after 30 days, and if you don't have credits left you'll get hit with that insertion without you ever approving it.  Then Final Value fees, fees on shipping, PayPal fees and by the time you're done the total adds up somewhere between 15%-17%...but divided into smaller hits so you're less likely to notice.      

2. Poshmark
The pros:  the listing form is short and sweet and the pre-paid shipping label definitely makes life easier!  good for clearing out good condition used items, but expect a barrage of ridiculously low offers as the user base tends to be younger & looking for bargains.  Not the site for higher end items, I'd personally stick to eBay or consignment shops like TheRealReal.  

The cons: Poshmark has a very time consuming social model.  They heavily encourage constant sharing, following, sending offers, joining events (parties, closet clear out events, list or lose etc, etc.)  Some of us have jobs already.  

The 20% fee is not justified considering their customer service is absolutely the worst of the bunch and they lag behind in features like offer auto-decline, or ability to keep your closet sorted as it shuffles every time you share.  Posh also doesn't support international shipping, a market that's arguably only needed by those who sell higher end designer brands. 

Overall, despite making it easier to list items Posh can easily consume hours of your day that don't necessarily translate into sales. 

3.  Mercari
In very many ways a lot like Poshmark with an interface/UI that feels outdated, but a much more reasonable 10% selling fee.  Lower selling fees mean there are definitely deals to be found here!  Mercari has more markets and is not limited to just clothing - although Posh is now trying to catch up.  I haven't had an issue on Mercari just yet, but from what I hear IF you're unlucky enough to need customer service, cut your losses and walk away. 

Both Mercari and Poshmark don't have a customer service number and communication over email is a slow, painful nightmare where canned replies and cut/paste answers after a significant wait are the norm. 

4. Facebook Market Place
I only recently started dabbling with selling on Facebook.  Sure, you don't have to pay anyone fees (except PayPal), but there's no protection for either the buyer or the seller.  This is the place to keep the biggest chunk of your sales, but also the highest risk of something going wrong. 

Others (still to come...)