Frankly speaking TE is not the right place to sell an Authentic iPhone with official bill & warranty at all.
I too own a iPhone 3G Purchased from Airtel in March 2010 for a price of Rs. 28,500 . When i wanted to sell it there were many comments of reducing the price to 15k,16k,18k . But truth be told people who want to pay that kind of price for a genuine product need to get a reality check. After all the OP is selling an original phone with bill and warranty that he purchased from the correct channel for 28k+ not some used grey market phone for 18k ! I am glad that i didn't sell my phone for that low a price, on 11th of Nov i got my perfectly working iPhone replaced from Apple (on the pretext of a hard Home button, which it wasn't BTW) for a brand new piece that too free of charge (that's what a real bill gets you). And didn't it feel good to hold a brand new piece after 8 months of using the first one. I also have another iPhone 3G which too shall be changed in the 11th month of its warranty.
Dear OP, do not sell your phone for a penny less than 20-23k as the people who are coaxing you to sell your phone are opportunists who are comparing your phone with iPhones which are 2 Years old & have non working buttons and severe cracks on their body. Trust me 90% of people would actually take this phone with bill & would get a shiny new piece replaced from Apple for themselves and make a grand killing on the low price that they would pay you.
To the people who think that they can get an iPhone for 13K-14K, do remember those are iPhones which are 1-2 years old with weak or non existent battery lives. Also they more often than not have hard/non working buttons & cracks on the body panel.
Best of luck for the sale :cool2: