There should be no problem at all if you are buying the kit, assembling it and reselling. You are not copying anything. You are buying wholesale, adding value and reselling.
The copying issue would only be about the software which is open source anyway and since the Guino assembly is the part you are selling which has the software on it already you are not copying anything.
The only thing that might happen is the kit maker might decide to make complete units himself and be in competition.
Donate a couple of bucks from each one and then you will have community goodwill as well.
To avoid a stockpile you could do them on order just wait till you have enough orders.