Anonymous wrote:OP here, following up in case anyone else ever stumbles upon this board with the same question. Sales tax and excise tax are the same - meaning you only pay one - and the dealer takes care of this for you (collects the sales tax and sends all necessary info to the DMV to get your registration completed). I believe if you purchase through a private transaction you'll then have to pay the excise tax when you register your car at the DC DMV but can't be 100% sure since that's not the route we went.
This is correct, you will pay excise tax upon registering your car unless you can show that it was already paid during the transaction. Many dealers will handle all this for you.