Re: complete lower??
One man's "complete" is another man's "complete minus furniture (or minus buttstock assembly)!!" I have seen this advertised both ways in terms of "complete" lowers but generally speaking, the seller or retailer makes it clear upfront what "complete" means in their terms. What were you "promised" or what was the wording of the ad/deal you bought the lower based on? If they said "complete" and left it at that, I think it is reasonable to expect a lower that is ready to have a complete upper attached and shoot...not one requiring an addt'l expense associated with or the installation of more parts.
HOWEVER...now to the unpopular portion of the response...with all that said, I think it falls on you, the buyer, to root out the details before buying. Sounds to me like you expected "complete" but had no idea what stock to even expect (collapsible/M4/etc., fixed/A2, etc.). You just expected it to have A STOCK, etc. The store owner, while saying "complete" meant something else. A short, 30-second conversation about the details of what specifically you were buying (or what you thought you were buying) would have saved you the trouble (i.e. - brand of lower, trigger type, grip, stock, etc.). The devil's always in the details.