I agree with what you're saying John, and it would be much better for potential buyers if threads didn't get relegated to page 2 so quickly. But why do sellers need as many as five?
I would suggest that the rule should say that nobody can open more than one fresh thread per week. i.e., a system where each seller would have one thread at a time, with a suitably informative title, and just keep updating it. Then periodically, he could start all over with a fresh thread, and he could include a link to his previous one if he wanted to. I would have thought that would be more helpful for potential buyers and would also be a much easier rule for you to police.
And personally I have to say that if the difficulty with one thread was that it gave Scant and Jez a problem, I would support letting them have a few more threads per week, as after all that might make their big ads easier for buyers to read. Some might say that's not fair, but I think it would be sensible and sensible is more important than fair.
It's a free service and it's excellent, so sellers should be very grateful for whatever they get.