Depends how old it is, what sort of condition it is in, whether the warranty is still good, how it overclocked for you...
Generally, see what others are selling it for, and undersell or match them if your part is similar.
75% of newegg is a good starting point. Putting OBO in your thread will leave you open to offers, you never know, you might get a trade offer you like, or a cash offer slightly below your asking price.
Some tips to help sell:
~ Good pictures of the item are important.
~ Screenshots of overclocking results, and links to any relevant threads on the specific part are good to include.
~ It is nice to offer a flat rate shipping, including shipping in the price makes transactions easier, and makes your sale more attractive to buyers.
~ Good references (heatware) will help you sell, and is a requirement to trade here on OCF (I see you have that covered
).
~ It is a good idea to provide a link or two to retailers selling the part that you are selling. Providing people with an idea of how much it is worth brand new will help you sell, especially if your asking price is much lower. This is also important in that it clarifies what you are selling - especially with video cards these days there are many "Light" models, or models with different features that drastically affect their sale value. Making clear what you are selling is important!
Thats about all I can think of.... Always pack your stuff in anti-static bags, bubblewrap (nothing beats it) and cardboard, in that order...