If the kit itself is defective, like missing parts or torn decals, you wouldn't be returning it to the retailer anyway, right? Unless it's his own product, the guy who sells it can't be responsible for problems inside the sealed box. For that, I always deal with the manufacturer, and I've had good luck that way.
Actually ( as a long time hobby store owner) you should contact the place where you got a kit first. Legit dealers all have accounts with hobby wholesalers or manufacturers where they buy their kits from and they are usually in contact with them on a daily/weekly basis. It's usually best to return a defective kit to where you bought it from rather than try to deal with the manufacturer direcly, in most cases. The store can supply a new kit to you and deal with the return/defective item himself.
That isnt to say you can't go to the manufacturer directly, but that often is much harder . Try getting parts from Trumpeter in China. They offer no customer service. Their US importer does not offer much support either to the general public. I used to get Trumpeter parts from their importer myself, as a store owner, since I bought my kits from them. I just contacted the importer/distributor and they sent me the replacements for my customers or to replace a kit I had to exchange.