In the $390 billion used-car market, a string of innovative business models have built on one another's advances to create full-service online auto merchants. Here are some of the ways businesses are disrupting the traditional methods of auto sales:

1. No-Haggle Pricing

CarMax Launched 1993

A nationwide chain of physical dealerships that offer no-haggle pricing and money-back guarantees.

2. Web Research

Carfax Launched 1984, went online 1996

TrueCar Launched 2005

Both help buyers research used cars online before visiting a dealer. Today, Edmunds, Consumer Reports, car dealers, and car companies all provide search and comparison tools and information about cars.

3. Messy Marketplace

eBay Motors Launched 1999

First site devoted to getting customers to buy a car, sight unseen, online.

4. Online Dealers

Carvana Launched 2013

Vroom Launched 2013

These online dealerships buy cars from sellers, inspect, market, and sell them online, and deliver them to buyers' homes. Both offer auto financing and post-purchase services.

5. The Uber Influence

Shift Launched 2014

Beepi Launch 2014

These peer-to-peer marketplaces connect buyers and sellers and own little to no inventory. Unlike previous marketplaces, they inspect cars, offer guaranteed minimum prices to sellers, and handle all marketing and transaction details. Shift offers home test drives and, unlike Beepi, takes possession of a seller's car immediately at listing.