Why can't the buyer/renter of the region build inside of it?
Make sure there is no build, block-break and block-place flags on the __global__ region. Remove those flags with /rg flag __global__ <flag> and instead set passthrough to deny with /rg flag __global__ passthrough deny. This change will make everything protected by default, while still allowing other regions to override protections.
Make sure that all other regions that are at the same spot of the AdvancedRegionMarket region have no build, block-break and block-place flags. Regions are protected by default, so these flags are not required and will cause issues.
Make sure that the AdvancedRegionMarket region has the highest priority (If flagGroups are enabled (config.yml), AdvancedRegionMarket sets the priority to 20 by default).
Make sure the walls/floor/ceiling of your region are not made of concrete. There is a bug in WorldGuard that prevents placing blocks against concrete, even it players are supposed to be able to do that.