There have been a couple of times recently when I've had a very odd experience on freelance marketplaces. I eventually figured out what was happening so wanted to warn others about it to.
I'd posted a job, filtered down the long list of candidates, interviewed and then used our disposable project method to arrive at the one I wanted to work with. Everything's peachy for the first few days and then I sense an odd shift.
Difficult to place exactly what tipped me off, but something had changed. My guy wasn't responding as quickly to messages and the quality of deliveries dropped. He seemed to chat differently. It seemed like a different person. Because it was.
To me, if I hire someone to do a job I want the same person I hired to do it or at least someone of the same calibre. The fact that my hire had swapped out someone else without telling me broke down my trust in them. So I fired them.
And then a lot of previous experiences made more sense. This wasn't the first time I'd had that strong feeling that something had shifted in quality.
In fact, many freelancer marketplaces account for this business model by offering experts listed as agencies vs. individuals. I can understand that. However my hires had all been individuals as I value building a relationship with experts.
I'd been baited and then switched. Baited by an excellent communicator and developer and then switched to someone poor who I hadn't signed up to work with.
This is a common practice so be wary and explicitly state that you only want to work with the person you're interviewing or ask them to be transparent with substitutes.