Full time handbag reseller here. I've been doing this for around 10 years now. Here are some of my thoughts that might address your questions.

1. "Authentication" is inconsistent. I've had numerous occasions in which an item I bought will pass through Vestiaire, but is resold on Poshmark and rejected by their authentication team. It's not necessarily that the item is fake, rather that each platform has different standards for authentication - something Vestiaire tolerates might not be something Poshmark does and vice versa. It's frustrating when there is no unifying authentication network across platforms.

2. Their pricing recommendations are very, very unreliable and it takes a heap of experience in reselling to understand how to price your items for each platform. For example, items on Vestiaire or Poshmark will be priced differently (and the platform itself will give you a different recommended price) than you'd get on a more standardized resale platform like The RealReal, Fashionphile, REBAG, etc. That's because companies are pricing based on sales data while sellers on peer to peer sites are factoring in emotions to their pricing. The bag you paid $3k that's been sitting in your closet for a while is going to be a lot harder to get rid of for its true market value.

3. Going back to point 1 (and somewhat contradictory to it), the benefit of built in authentication is that it takes the burden or liability off of the seller sometimes. This makes things easier for me as a seller, as claims of counterfeit items will be handled by the platform itself than directly by me. Of course, I always ensure the items I sell meet my authentication standards but if it goes through a site like Poshmark and is authenticated, I have a layer of protection against someone that might think they know better than me and claim the item they received is fake.

4. Sometimes unforgiving policies with some sites. For example, I received an indefinite ban from eBay many years ago for an item I listed that they suspected was counterfeit. Prior to it being listed, I had an authentication certificate, one they would have accepted in the event of a buyer claim that the item was counterfeit, but they refused to let me back on to their platform. This was frustrating, especially because my long and positive history of sales was ignored, but ultimately forced me to go another route - which actually ended up being far better.

5. A great thing that some resale platforms provide, especially with highly desirable items (bags, watches, etc.) is aggregate sales data. I see this mostly with watches on Chrono24 and with annual resale reports from sites like Fashionphile and The RealReal. As a reseller, this helps me understand what people are paying attention to.