You could take XP in your bar-thingy-majig-thing and right-click while holding an empty glass bottle, it would turn the glass bottle, and xp into an XP bottle, this would be a way of "selling XP" but you could use the xp from a grinder too. Remember, just a suggestion!
I like the idea, as long as the amount of xp bottled and lost is more than what you get from the bottle. Bottles vary, right?
A major problem: when attempting to fill water bottles, you would probably mess up and fill it with xp, forcing you to use another bottle or break the xp one. Also, it's not really vanilla, and you could just pay someone to enchant for you. Not too convenient, but does the enchanting system really need to be more convenient?
I'm not really sure I like this one... if you really want to sell xp you can easily enchant things and sell them with it, so I also don't quite see a point to this either, especially since it's easy to get them from villagers as well.
This system can be very easily abused. XP is shared cross server so any1 could sit at a mob grinder in SMP1 then go to SMP2 to change it into bottles. they would never have to leave the mob grinder.
emm, no it's the exact same, it's just basically selling XP, of coure EVERY suggestion needs tweaking, but you already do that with enchantments.
Three more problems: 1. It would most likely create a sharp rise in the price of glass/sand, since once you use the xp bottle you can't get the glass back. 2. It would, to some degree, invalidate the TEXP system, since players could level up to, say, level 30, bottle all of their xp, then just re-use it. 3. Someone (the staff) would have to code in a way for the xp bottled to equal the xp lost. Unless you want to add a new xp bottle, you'd have to make it so that right-clicking with the empty bottle takes a random amount of xp, and fills the bottle with the same amount.
1 Thats good! We need an economy boost. They could just make it so that xp bottles dont count towards TEXP