Well-versed Development Manager, Project Manager / Scrum Master & Consultant for Startups, Web, Mobile and Software Apps dracopol.com
Such platforms cost tens of millions to build, maintain and especially advertise. Even i you build one, the marketing budget should be quite high.
You also need something to differentiate them, like for example Clarity is different from Upwork due to consulting focus.
If you still want to do this. start doing some specs, have them reviewed by a system architect to chose a technology that fits and to estimate the development hours than put together a team and build it
Please let me know if you need any help, .i have a solid experience in building platforms from scratch.
Eastern European and Russian devs are very appreciated on remote on-demand markets like Upwork.com, for example. I personally hired a lot of programmers from Russia to work for US startups.
If you have a good portfolio, these kind of platforms have all the means to mitigate the payment risks - by intermediating the payment - until you'll get a good profile there with cool feedback and ratings.
No need for marketing and going there, just do your job well.
This is how you can get on that market.