The past tense of the verb "buy" is simply "bought".
This is a common error that people make, particularly when trying to use the past tense of verbs that end in "-y". However, the rule is that you remove the "-y" and add "-ied" when forming the past tense of verbs that end in "-y" preceded by a consonant.
Here are some examples:
* Buy: bought
* Cry: cried
* Try: tried
* Fry: fried
Let me know if you have any other grammar questions! 😊