InfiniteDivide wrote in post #16667663
I agree, I have never sold a lens and then re bought it. The money I have slowly been putting into my lenses is safe to just sit there.
For others, I totally understand freeing up the cash to buy a different lens that you will use more; but to buy the same lens over and over?
Would your question change if you could make money each time you bought and sold? 
I buy a lens, use it for a season, and then sell it. Later, my formats may change or needs may change, so I buy that lens again. This protects me from depreciation costs, as well as builds up funds for a different lens later, and I get to use any lens for a period of time. This is a more cost effective way to renting a lens. If I use a lens extensively, I keep it. If I find a lens that isn't being used for a long period of time, I sell it, and I don't lose anything in the process.
I just bought a lens a couple of weeks ago, and tried it out for a week. I decided with my 50 1.4 and my 100 2.8, this new lens wouldn't see much use. I listed it on Amazon this week, and made $70 selling it there. Just an example.
In summary, if you buy lenses and then sell them later, and lose money each time, you might want to change your tactics a bit. There is almost no reason to lose money doing this.