I don't know if this changed, but didn't Tesla used to use Autonation for trade-ins? Anyways, as with trade-ins, even if Tesla does buybacks, you aren't going to get a very good offer compared to private party. It even says so in the trade-in page.
http://www.teslamotors.com/support/trade-ins
If you don't want to deal with private party, you can try autonation and carmax. You can try calling your local Tesla store to ask also.
However, if you are talking about the buyback protection policy, it does not apply unless you use their financing (makes sense given buyback is designed to be an answer to leasing; if you use your own financing it's no different to Tesla than you buying the car outright).