While the concept is excellent, the implementation is terrible. Here's what I've encountered since mid-Nov.
Here are the problems that I've had since I started using sellbrite:
1. I had three items in inventory and sold 2 through Etsy, sellbrite set my inventory to 0 on all sales channels until the order was fulfilled (I've confirmed other variations as well). Once the order was filled, the inventory was set to 1 as it should have been. Being in the software industry, I troubleshot the issue and explained the problem to customer support in great depth. Their solution to troubleshooting was to give them account access to my Shopify store, which is ludicrous.
This issue is still unresolved.
2. When I signed-up for sellbrite, I entered the promo code to lock in at $20/mo, when my bill was coming up, I received an invoice stating I would be billed $49. I reached out to confirm that I would only be billed $20 and indicated that this was confusing. Here's the response I got: Yes, I completely understand that. "That promotion email is handled by our billing service. We asked for them to make this improvement, but it's something that they have not fixed on their end." Not an actual issue, but very confusing.
3. In the reports, there are columns for the sales channels, including Shopify. I thought that was cool, I'd be able to see Shopify, and all sales channel reports in a single location. That's not the case. Shopify isn't populated, it's just included in the UI. Again, this is misleading and confusing.
4. I received an order through Etsy, and it routed to Shopify through Sellbrite. I fulfilled the order through Shopify (as sellbrite instructs to do) and purchased First Class with Tracking. The tracking number was not successfully passed to Etsy via sellbrite. I troubleshot the issue and finally manually finalized the transaction and insert the tracking number in my Etsy shop UI to resolve the issue (this has happened multiple times).
This is still an open issue.
5. I've made changes to some product variants on my Shopify site. I kept a couple of variants, deleted a couple and added a couple for one of my products. Sellbrite is not recognizing the changes, and is therefore not syncing with Etsy or Amazon. It's setting my inventory to 0 on those sites for the variants that I didn't change.

I've reported all issues to sellbrite, and not to toot my own horn. Still, the only way I've been able to work through any problems was due to my technical knowledge and persistence. Unfortunately, there are limitations, so I can't resolve or workaround every issue I encounter.

they are full of crap,i was going to sign up for the 20/month plan but as soon as i was about to put my credit card info in the fineprint before putting my card info it say i will be charge 49/month so be careful when signing up their prices change.

UPDATE ok iam updating my review because they responded fast but i guess it was a miss understanding i'm going to try it for the 14 days and if everything goes well than i will give 5 stars.

Please don't be fooled by all the positive reviews. This software has so many problems and issues with the connection between website and marketplaces. Not to mention you will have to wait 24-48 hours to get basic support. If they can't resolve the problem you simply won't hear back from them. If you are a very small company with few SKU's this is a perfect software for you.