As far as editing sold vehicles and replacing with live inventory, that is absolutely something that our company (AutoSweet) does (and a couple of the others as well). We poll inventory daily so we can update pricing, add photos, etc. as it is, so identifying sold inventory, editing the post, dropping the sold vehicle and replacing with a live one is part of that.
There are ways that you can attempt to do better with search optimization, we are constantly making changes via trial and error to do that, but as others have said...there is no silver bullet for it and what works this month may not work next.
We also don't believe there's magic times for magic types of vehicles, we just think you need to post daily so you have something on the front page when its sorted by newest (as much for branding as anything, we create image wraps for our clients). Search optimizing to the best of our ability for vehicle type or manufacturer is our biggest focus, those are the most logical common searches we believe, and we do pretty well on that consistently.
We also believe offering a text option is important, using local area code tracking numbers unique for each market you're posting in so you can track results/ROI by market, and one of the biggest is using links (although not click-able) to go to the VDP on your dealer site in a way that the traffic is visible in Google Analytics.
There are some excellent competitors out there, but anyone claiming the things I have seen in this thread should be an indication they aren't one of them.
We will be at Digital Dealer in Tampa, NIADA in LV, and Digital Dealer in LV this year, you can come by and see what we do in person or call any time.