Gilbuying is probably summarized by
the 80/20 rule (link).
In other words, the top 20% of gilbuyers account for 80% of the business.
The other 80% of gilbuyers are the casual buyers who, combined, only account of 20% of the business.
Pure speculation, but it does apply to lot of things...and it often holds true for luxury purchases in particular.
Edit: And my point, which might not have been clear, is that even though I am a HUGE proponent of banning gilbuyers, I suspect that merely eliminating the casuals will not amount to a huge dent against RMT.