Basically there are three possibilities depending on the condition of your garments - throw away, donate or sell. As a rule of thumb - the better the condition, the bigger the chance you can make some money by selling something!
Let's concentrate on the selling, because it never hurts to get at least a few bucks back for your sometimes not-so-wise shopping decisions. As the selling process can be somewhat tedious, let this be a warning for future purchases too. You snapped up something in a heartbeat and now it takes months for anyone to be interested in buying that item despite it being brand-new with tags. No one said this would be easy (or fair).
Some might not want to go through all the hassle all by themselves and choose a consignment store, others will set up a table on a flea market or do a yard sale, and then there are those who will resort to selling online. There are (dis)advantages to all of these options and it's a bit of a trial and error kind of thing to find the perfect spot for yourself. Personally I only sell online, though not through Ebay as I prefer to set my own prices and dislike the idea of having to pay fees.
Good descriptions and photos are key to selling. I find nothing more irritating than one-liners that read like, "Red Dress from Esprit, Size M". This tells you next to nothing. Buyers need to know the exact condition and measurements to make a decision. While it doesn't hurt to praise the loveliness of a garment to set the shopping mood, don't skip details about flaws. People won't necessarily mind if they have to sew on a loose button, unless you forgot to mention it, that is.
3. Don't expect miracles.
Clothes are hard to sell. Not the right size, length, color, you name it. I've sold garments within a day and then it took over a year to find a buyer. You need a whole lot of patience and/or sell your stuff for next to nothing if you want to get rid of it rather sooner than later.
You will make some bucks, all right, but just because you offer a new pair of pants that cost € 80,-doesn't mean anyone will be willing to shell out more then, say, € 10,- for it now. While I always welcome a big of haggling, I have encountered a fair share of impertinent potential buyers who'd prefer if I gave things away for free. If I wanted to do that, I'd donate.
5. Let it be a lesson to you.
Think about all the effort that goes into selling things you just had to have, that never quite fit you anyway, but were on offer for a price that was just too hard to pass by and ultimately leaves you with a big monetary loss when you finally jump over your own shadow and try to find a new home (or rather, wearer) for it. Next time you feel the urge to buy something, or anything, really, think long and hard about the ugly world of reselling.
How about sharing a bit about my own adventures in selling "old stuff"?
I don't just sell clothes, but plenty of other items too. So I don't plan to read a book again? Let's sell it. That DVD Box was nice to watch, but you won't ever do so again? Out it goes. The perfume you received as a gift is too flowery for you after all? Find a buyer for it. Those coupons you won for a restaurant that won't even serve vegetarian food? Away, I say. Jewelry you'll never wear again? Look for a magpie with money on their hands. And so it goes ...
This year alone I earned more than € 800,- with the slowest month in February when I cashed in about € 47,- and the best month being March when I earned € 230,-. Apparently I have a whole lot of stuff I wanted (and did) get rid of.
Now that you purged your closet, and with a little bit of advice from yours truly, you're all set for the big selling-game! Wish you luck!