I once used DVLA's website to check the availability of a certain number and it came back as unavailable. I thought that meant it had been issued, but some months later it came up for auction on the DVLA website. I guess that if DVLA think it has any value they will auction it and get as much as they can. One tip for you regarding "plate dealers". If you see a number on a dealers website, check other dealers for the same number and very often you will find it at a lower price. Also check DVLA's auction site.