It really depends - marketplaces like Sedo are emphasizing fixed price sales these days - and using SedoMLS which gives you expanded exposure (at an increased commission level).
If you choose to park elsewhere, many services allow you to add a for-sale message and handle it on your own, letting you choose how you want to facilitate your sale.
You'll need to do the math and see what makes more sense for you. For example Sedo has a minimum fee so it won't make sense for lower priced domains, especially for some cctlds.