Yes you can sell guildpoints. When you hit 71 you make a topic is the LS trading forum saying like guildpoints for sale.
If you're selling guild points, you must make it clear in your post what class you are and how many GPS you will get for a pass/fail.
If you're offering, be sure to make it clear if you're offering FG on each GP earned, or on a pass/fail.
A mediator will hold the amount of FG required for a pass. The person joins the guild, attempts the MQ, and receives FG from mediator.
If it's a fail mediator will refund the required amount of FG to the guild owner.
And about the mutables, after the update mutables have lost most of their value so you will not be able to get 500fg for one mutable