newegg lost some of my business recently after thinking it was a good idea to make me fill in a captcha before taking my money.
Probably they are combatting fraud, especially the variant "check if the credit card is still valid". There's not much defense against a botnet operator trying out a 100k dataset of stolen CC numbers other than captchas :(
For the interested, this kind of fraud simply orders cheap (on the order of 1-2$) stuff online to check if the card/cvv is valid. Doesn't draw much attention unless one of the victims has transaction notification active or diligently checks their CC bill.
In my experience, buster also works better when you don't use the Google Speech API but any of the other ones so Google can't correlate.