Man, forget a merit point shop. Give me the Vampire Black Market from Wild Arms V where you buy stuff with straight-up experience levels.
Two rules:
1) You cannot "bank" levels. Anything you buy will be extracted from your current job's levels at the moment of purchase.
2) You can buy up to as many things as you have levels for at that moment (e.g. 9 10-level items, 3 30-level items, 1 98-level super item, etc.) but you must be on a level 99 job to initiate the transaction, to avoid gaming it with low-level jobs.
There are a TON of things you could do with this.