It is always just finding the right kitty for the right buyer. Because of that, I don't know of a way to get rid of 1000 boxes at one time. It is more one box at a time kind of thing. Anytime I've seen someone try to sell a grouping of kitties, they never got a good price for it even when it was collectibles.
While, I agree, new traits sell faster because breeders try to get them when they don't have them, I like to sell older furs. It's like the difference between the sales of pop songs versus R&B. There are people still buying R&B, and just like R&B singers, I find you have to find the right place for the older furs and eyes. I'd suggest going to a few auctions. Listen to the auctioneers -- some of them like certain furs or encourage buyers to buy retired traits. One auctioneer calls them couture cats if the comibination is pleasing. Another auctioneer loves matchy matchy cats which have eyes that match the fur color. I find I don't sell well at daytime auctions ranging from 25%-50% of the time I sell, doesn't matter who the auctioneers are as it seems more daytime auctions have just the breeders come. Night time auctions I sell 75-90% of the time. Night time seems to have a greater range of buyers who come to the auction.
Also I do well on bidboards on one particular sim -- it isn't a big sim but the live table allows people to put up kitties from 0%-100%, not just 100%. The live table which brings in the traffic are close to the bidboards and it seems like there is a variety of buyers as I've sold Maine coon, Bali Seal and Bengal black. My experience on the big sim bidboards, those don't sell well but I can sell my new traits. Some sims require you to have a shop there to use the bidboard but others don't. So browse bidboards at several sims -- check out what sells (best to check close to end of auction time because a lot of people don't bid until the last 10 minutes) -- and look for bidboards that have the type of kitty you want to create/sell is selling. Some bidboards are cheap 1L-20L and some are more expensive 75L.
also you don't know why people buy. A fair amount of people have a nostalgic fondness for the first fur/s they bred and will sometimes buy that fur again if it comes with an eye combo they don't have. Sometimes people are looking for a particular eye or fur for various reasons. I went through a phase where I was trying to get every single eye and did so although I had to pay a premium on a few of them because they were hard to find. I am currently working through the list of furs now. And I am pretty sure I'm not the only one. Sometimes someone wants a cat that looks like their RL cat. Another person might be buying a gift for an SL friend who doesn't have a kittycat. Breeders buy what they think they can sell high but everybody else buys for a multitude of reasons.
I've sold my older fur/eye combo for between 700-1100 at auctions and bidboards. It's not the premium prices of onyx or tonki but it's okay and does net me between 75L-100L per trait. I'd also suggest uptraiting a few so that they have swanky tail or light wave whiskers or something on the recessive end: it gives a bit of style to the cat and introduces uniqueness in the combos. Mysterious/curious/plush brings the price down because so much of those traits is available out there == think mass production versus "couture". Cats with mysterious tail/curious ears/white mysterious whiskers combos are so ubiquitious that 100L-350L is about what they are worth. Mass production where a lot of something is available brings down the prices of the mass-produced poduct just like RL. Adding a few traits like dreamy can add pop to a kitty. I've seen one person do that and her kitties sell regularly for 1,500-2,000 at auction even though they have an older fur. She makes lovely combos with some of the traits that were released a year ago but still fairly recessive.
My other bit of advice is watch the Monthly Cat contest of kittycats. A lot of times it will be an older fur/eye combo with older traits too as the contest seems to be set up to be fair to newbies as well as breeders. Try to make that cat. If you can then you can earn anywhere from an emerald to a diamond. If you have an older cattery, you are set up to have a chance at winning a jewel which you can sell or save to get a fancie cat. Like this month's cat:
Fur: Snowshoe - Chocolate
Eyes: Fire (Shape: Curious | Pupil: Big)
Shade: Natural
Tail: Plush
Ears: Genesis
Whiskers: Silver (Shape: Guitar)
http://kittycats.biz/forum/showthread.php?tid=20127
So it's not really advice on getting rid of a cattery but may help in selling individual cats that have older furs/eyes. Anyways, I hope it helps.