Jason,
I'm always very curious about numbers so I compile a lot of stuff and try to make sense of it so I'll share some data that may help you on this 3rd party site stuff.
The data that we measured is from Craigslist but for the purpose of my analysis I think it works for any website generating leads.
Recently we were able to track ad opens in CL (someone clicks open the ad) then click on the ad (so customer goes to website).
V- views (an ad was clicked on and fully opened) C- Customer clicked on the ad and went to dealer's website L- lead (phone call or email. Only from the ad, we lose tracking once customer is on website or chats).
[FONT="]1. Dodge dealer large metro area[/FONT][FONT="][/FONT]
[FONT="]V-76,358 C-5,349 L-240[/FONT]
[FONT="]on Avg, 14 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 22 Clicks are needed to generate 1 Lead[/FONT]
[FONT="] [/FONT]
[FONT="]2. Honda dealer large metro area[/FONT][FONT="][/FONT]
[FONT="]V-70,719 C-3,172 L-119[/FONT]
[FONT="]on Avg, 22 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 27 Clicks are needed to generate 1 Lead[/FONT]
[FONT="] [/FONT]
[FONT="]3. Cadillac dealer outside metro[/FONT][FONT="][/FONT]
[FONT="]V-38,808 C-2,262 L-84[/FONT]
[FONT="]on Avg, 17 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 27 Clicks are needed to generate 1 Lead[/FONT]
[FONT="] [/FONT]
[FONT="]4. Independent outside metro[/FONT][FONT="][/FONT]
[FONT="]V-13,396 C-428 L-100[/FONT]
[FONT="]on Avg, 31 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 4 Clicks are needed to generate 1 Lead[/FONT]
[FONT="] [/FONT]
[FONT="]5. Independent in metro area[/FONT][FONT="][/FONT]
[FONT="]V-73,472 C-2,786 L-117[/FONT]
[FONT="]on Avg, 26 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 24 Clicks are needed to generate 1 Lead[/FONT]
[FONT="] [/FONT]
[FONT="]6. Independent in metro area[/FONT][FONT="][/FONT]
[FONT="]V-30,495 C-1,726 L-73[/FONT]
[FONT="]on Avg, 18 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 24 Views are needed to generate 1 Lead[/FONT]
[FONT="] [/FONT]
[FONT="]7. Independent outside metro area[/FONT][FONT="][/FONT]
[FONT="]V-8,822 C-521 L-47[/FONT]
[FONT="]on Avg, 17 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 11 Clicks are needed to generate 1 Lead[/FONT]
[FONT="] [/FONT]
[FONT="]8. Audi dealer outside metro area[/FONT][FONT="][/FONT]
[FONT="]V-31,923 C-2,492 L-71[/FONT]
[FONT="]on Avg, 12 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 35 Clicks are needed to generate 1 Lead[/FONT]
[FONT="] [/FONT]
[FONT="]9. Toyota dealer in large metro area[/FONT][FONT="][/FONT]
[FONT="]V-36,470 C-2,817 L-95[/FONT]
[FONT="]on Avg, 13 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 30 Clicks are needed to generate 1 Lead[/FONT]
[FONT="] [/FONT]
[FONT="]10. Independet dealer in metro area[/FONT][FONT="][/FONT]
[FONT="]V-17,312 C-1,328 L-132[/FONT]
[FONT="]on Avg, 13 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 10 Clicks are needed to generate 1 Lead[/FONT]
[FONT="] [/FONT]
[FONT="] [/FONT]
[FONT="]OVERALL AVERAGE[/FONT][FONT="][/FONT]
[FONT="]V-397,775 C-22,881 L-1,078[/FONT]
[FONT="]on Avg, 17 Views are needed to generate 1 Click[/FONT]
[FONT="]on Avg, 21 Clicks are needed to generate 1 Lead[/FONT]
Autotrader, Cars.com, etc numbers are probably very similar to these numbers. So take dealer #7 for example. If their closing ratio is 8% they may say that this website sucks because they sold 2-3 cars last month from it out of the leads they were able to measure. What about the almost 10,000 people that saw their ad, name, URL, etc that drove to the dealer or Google them a day later?
I'm not inferring that Autotrader works or doesn't based on my numbers but that what we can measure is a small piece of what online advertising provides.