The ModularGrid Marketplace is an area where private persons can trade their modules.
Besides this tabular list you can also search for all offered modules in the Module Browser just by setting a region in the Available in Marketplace select box.
Read more about How it works? and also see the thread Good Traders on the Marketplace.
Currently there are no offers on the Marketplace for Frac modules. You can offer a module by browsing to the detail page of a module you like to sell.