This is a new property that has been added to workspace. This is a feature that is intended to be used for end-user protection.
Most games do not need to sell third-party assets – assets that were not created by either Roblox or the place creator. However, places that are trade hangouts and certain group/clan places do sell such assets.
Setting “AllowThirdPartySales” to true will enable the sale of these assets. The default value is false, thus this is an opt-out feature. (Non-place-creator, non-Roblox assets cannot be sold inside of a place by default.)
This property will be displayed in RobloxStudio after this release, however there will be some amount of time before the feature is actually enabled. This is to allow developers some time to update their places.