Is buying bonus allowed during a deposit bonus?
No.
During a wager or a deposit bonus it is not allowed to buy bonus, as a single bonus buy will violate the terms of max bet and your bonus will be voided.
4.12. The maximum permitted bet amount while a bonus is pending or actively being turned over is $5 USD.
For example a 10$ bonus buy is considered a 10$ bet, and not 0.1$, so always keep that in mind.
⚠️ Please note that this is just an article talking about the deposit bonus, and it is not the final rule maker and it can get outdated. You should always read the TOS of the rollover for the latest rules.