Eternal Arena – The 7th rune slot costs 600 points. I have enough points, but when I try to purchase the slot, it tells me I need 1000 points.

You can’t exchange anything with the points in the guild shop until you have 1000 points.  

Also, if you leave a guild and join a new guild, you cannot make use of your points until you make 1000 points in the NEW guild.