You will get the best price with a private sale to someone who is knowledgeable on VW's. Any auto dealer is going to look the car up in the resale database and see it as a max of $5,000 (go check blue-book online). The biggest hit on the resale is that it's a TDI, and with the VW diesel issues dealers don't want to mess with them.
There's recently been a story on this forum of someone who got a great deal on a TDI because the dealer took it as a trade in without knowing the engine type. Then priced it to dump it when he found out.
Check Auto Trader.com to see similar cars for sale. I'd suspect a good price to a private seller would be between $18,000 and $28,000.