I can give you some suggestions based on my perspective and my needs, you can extrapolate on this as I imagine my needs will match a % of your potential client base. Especially since some of the feature that could interest me is not something I've ever found on another website offering the same kind of service you want to provide.

Here is what I am:

+ English speaker, I do not speak japanese and do not wish to learn.
+ Game collector, strong focus on NTSC english games.
+ Curiosity towards NTSC-J, % of my collection dedicated to NTSC-J is < 1%
+ Ignorant as to what console have native NTSC-J support and what games have dual language built-in OR games that are english friendly (close to no story, english menu, strong emphasis on gameplay)

Here is what I want:

+ A website that will educate me as to which console can play NTSC-J natively, and what are my alternative (for example I know the SNES is NTSC-J friendly and only need to have two plastic tab removed, tell me that!), also sell me NTSC-J consoles!
+ A website that will identify what games are english-speaker friendly. If it's a game with close to no text and is all about gameplay, yeah I am interested! I just probably don't know about that game, tell me about it and you'll create a need I did not have before (good example would be Parodius Da! for SNES, that game is awesome and you don't need to be japanese to enjoy it, there are probably a lot more game that fit in this category)
+ Competitive pricing. If I can find your product cheaper on ebay, I will buy it on ebay.
+ Paypal, it's simple, it's efficient, don't make me register on another paying service.
+ Picture of every item (stock photo is fine), a youtube link to a gameplay video (that's hot), and a description of the product that is not the useless text found on the back of the box.
+ Good inventory. Going to an online game store and finding that everything is out of stock sucks balls. Your idea of having a request to stock that will be fulfilled within 48h is very cool, if you can deliver on this.

If you manage to create an online store that incorporate all of this, I will give you my money!