I am in the early stages of planning an online business which will bring together those who want X, and those who can provide X.
Looking at Apple's App store model and Amazon's Publisher model, they each charge a hefty percentage of a sale, no doubt to cover the cost of running the system, and also to cover payment processing fees.
Would it be sensible to follow this model?
I don't know what makes more sense, to charge for example 20% plus processing fee, or something like 30% flat fee.
The question was "is 20-30% a standard, accepted practice". The answer, frankly, is no. There is no standard pricing practice for service providers. What you charge should be determined by several factors, such as:
The primary determinant of what you charge should be "What is your customer willing to pay?"
If you don't get this right, you have no customers.