GFY Sell & Buy Forum is the best place to sell in my opinion. You could try other adult webmaster forums as well. Mainstream auction sites don't really have the demand for these sites, nor allow adult listings.
As for a safe way to handle the transaction, I also had no issues with using Escrow.com for adult, and I recommend it.
By 3.5, what did you mean, months, or years? In my opinion, 3.5 months is too low, and 3.5 years is too high.
Also, there is no such thing as the "the usual 3 months earnings" that a poster above mentioned.