As you are asking about Treasure Hunter Keys, thread has been moved to the Treasure Hunter Forum.
There are limits on buying keys. Here are J Mod responses from similar past threads:
Original message details are unavailable. I understand you've been having issues with your buying limit having been reached for Treasure Hunter Keys. Limits for purchases are based on a number of factors. I am unable to go into detail on what these factor are, as truthfully I don't know. What I do know is that previous purchases are taken into consideration but it is our systems that sets what quantities you are allowed to buy in.
I am also unaware of when this limit will be lifted as it depends on a number of factors that the system decides but hopefully it will not be too long. Original message details are unavailable. As I mentioned, the system sets these limits, unless you are buying through a third party such as BOKU, who set their own. This is not a widespread issue and only impacts very few players, as it usually takes quite a lot of purchases, to trigger the limit being imposed.
Please be aware that the limits are there for the best interests of all parties and I hope it hasn't impacted on your gaming too much, but the limits will not be lifted until the system makes that decision.