How can selling your SUV save the world any gas? You must be assuming that the new owner will drive fewer miles than the old owner. Where is the evidence that used car buyers drive less than new car buyers? The solution to the SUV problem is not buying new ones.
Right, so if you flood the market with used SUVs, that will naturally make new SUVs less appealing - they get recycled.

But what I was really suggesting was that if individuals want to be less exposed to higher gas prices, those are ideas on how to reduce it, but it works at a national/global level too.

If you sell your SUV, the buyer may be one who was considering a new SUV. Also, if the market is flooded with SUVs, this drives down the resale value of SUVs, and indirectly, the amount people are willing to pay for new SUVs, thus decreasing the profit auto manufacturers realize from SUVs, thus prompting them (maybe) to build fewer SUVs and to build a greater number of fuel-efficient vehicles.

Agreed, the effect is small, but if enough people do it, it is there.