Several other users have also already reported issues related to the EcoItems plugin with trades not working. There already have been compatibility issues with the other Eco plugins in the past (see https://github.com/Shopkeepers/Shopkeepers-Wiki/wiki/Known-Issues). I don't know whether the EcoItems issue is related to those or caused by something different. However, after having already spent too much time keeping track of the Eco-related compatibility issues mentioned there, I won't spent any further time debugging this or any other Eco-plugin related issues. Your best bet is to inform the Eco plugin developer about the problem so that he can hopefully look into the issue.