It's actually Tibia that does this. Right click on the trade window and you'll be able to change the setting that prevents you from selling your equipment.

Imo the real problem is that Self.ShopGetItemSaleCount() reports your equipment as sellable regardless of your trading settings. Because of this people get stuck in infinite loops whenever they try to sell something that they are also wearing. This can be worked around in lua, but almost no scripts do it.