I've never heard of 10 slots. As far as I know, the max is 7. I suggest that if you'd like to sell more items while your AH is full, just open up a Bazaar and advertise a bit by shouting now and then what the best items you have are. ^^
Want more slots do like me and make more chars as long as you don't mind spending an extra buck per content id. I keep one char at home tending to the garderning and use that char as extra slots incase i run low.
This would be an excellent quest for an expansion, in my oppinion. Just like your Mog Safe size, or your inventory size, give a quest that you could expand your AH slots to 10... or more!