In order to set or change your product's condition in Linnworks.net, you need to adjust it in the eBay configurator's main settings.
Go to Settings > eBay Configurators > Open the Main Settings window:
Item condition can be set as a default value or via the extended property. Please note, extended properties always have priority over the default value, more details are here.
Item condition is a default condition that will be sent to listings.
If you select a particular extended property from the drop-down of the Extended property for condition, for example, eBay Condition extended property, then Linnworks will send to eBay the condition that is set as a value of this extended property for every particular item. Condition value should be item conditions IDs from eBay, you can check them in this guide. Good example would be this: