I am not interested in buying it -- they are generally a single person use item?
But, I ran into the same problem with mine. You may not be using the correct size breast sheild - the plastic part that touches you. They come in sizes -- I know that babies r us carries the next size up- large. There is an extra large available too -- you can go the medela web site for that. Also at the web site it shows what the pump should look like in the video when it is working correctly.
Just thought you may want to try that prior to selling it? Also, at breastfeedingworld.com -- they have lots of products to make using a pump on a regular basis easier -- I used my pump for 6 months with a hands free pumping bra and larger breast sheilds.
I am passing on advice that was given to me by a friend of mine -- because didn't want to see the 200 plus I spent go to waste.