I like this question frankly eventhough the answer might not be simple or the approach might be different depending on the individual breeder's taste

I noticed a tendency on the market people are looking either for new hidden or shown traits or high traited cats.
The reason people like to buy high traited cats is because they know when they breed such high traiter with any cat or even a starter it is at least packed with goodies.
So what I would do in your case, because since gender is revealed on the box I notice people are even more picky, is invest in some well traited cats and let them make some offsprings and include them in your other projects and eventually back breed or sibling breed them to get the unrevealed hidden traits from them.
In example two 9 traiter kitties make always 9 traiter offsprings so this could really value up your kitties.
The prices for high traiters vary but in my opinion they are always valuable.
And indeed if you have managed to make something to show for I would contact a manager of one of the excisting markets to put you on the waitinglist, if there is any, for either a small venue or a bigger venue.
I noticed also a nice shop layout helps to sell kitties why I prefer to rent some more prims than I stricktly need to display my cats to make it more attractive with decoration and such.
When the market does well the price to rent is easily covered and a well balanced and decorated venue attracts the eye of the potential buyer.
Having said that the markt knows fluctuations ofcourse, such as during the summer season it is usually somewhat slower, which makes the waitinglist also shorter to find a new and good location as well, so this is a good time to find a nice shop I think.
This forum and various in world groups are an excellent place to promote your best kitties to bring traffic and draw people's attention.
Most markets have their own groups in which you can promote the kitties displayed there and there are also groups which allow every kitty to be promoted no matter on what market they are.
Also a nice way is to show what kitties you have is make a thread on this forum under the section showroom with pictures of your kitties and open a FlickR account if you don't have one.
KittyCatS has an official
KittyCatS FlickR group as well where you can post your kitties.
If people like what they see they can for example visit your store via either your in world picks if you have your store listed or via the signature you can make here on the forum with a SLurl.
Now gender is revealed on the box it is indeed as you suggested an option to sell boxes at marketplace eventhough I'm not sure if the community is using this option already.
I personally prefer to get involved with a market not only do they take care of the promotion of their sim(s) so you don't have to worry about that but I noticed also it is nice because markets are often a place where people meet eachother during auctions and other social events such as breeding classes.
In the lobby of the KittyCatS Main Store you can find also a free merchants pack with all sorts of cute signs for your store.
You need to wear you Addicts grouptag to get it.
I hope this helped some good luck with selling it is so much fun