You have a point there! I didn't think towards that direction. it is really possible to buy a service on one freelancing site and sell it at the another freelancing sites without having a knowledge about it and you will make good profit.I don't think it is practically impossible for you to make money of freelancing website if you don't have a skill. There are some people who do not have skills but the act as middlemen between clients and freelancers and they make commissions at the end of the day.
I think as time goes on a lot of freelancing platform that pays in cryptocurrencies will actually come up. The cryptocurrency adoption is increasing every day.