I work through Arc.dev [0] and I like them. From what I understand Arc asks no commission, so I am not sure how they actually make money. You set your own rates and that's what clients pay. Perhaps clients pay a one-time fee upon finding a suitable contractor?
There doesn't seem to be a race to the bottom on Arc.dev with regards to rates. There's a vetting process, but not as difficult as TopTal imo.
Arc takes ~15-30%, they just take it from the client's side instead of the freelancer; but your rate isn't the one the client is paying.
I do like them a lot though and have gotten clients through them (and codementor; though that last one is, too, getting polluted with very-low-level coders just randomly accepting jobs they do not have the skills for; having used it as a client recently, I've been less impressed)
There doesn't seem to be a race to the bottom on Arc.dev with regards to rates. There's a vetting process, but not as difficult as TopTal imo.
You can get an idea on the rates of freelancers on Arc by reading the following: https://arc.dev/remote-developer-salary
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[0]: https://arc.dev