I was approached by a good friend a few years ago to do this with some company in California. I was intrigued and eager to make a lot of $ each month by doing this. (I know of someone who makes north of 100k a year doing this with very minimal work). I asked the opinion of a lawyer I trust who advised me not to do this. He said the biggest risk would be if one of the authorized user (who's goal here is to "artificially inflate" their credit score through your credit line) goes to take out a loan with their "new" positive credit and then defaults (probably for the same reason they initially needed this service and are forced to pay big $ for this service as they ARE NOT USUALLY TOO CREDIT TRUSTWORTHY AND RESPONSIBLE) the bank may go after them for fraud (misrepresentation of credit worthiness as they paid to artificially inflate their credit with the sole purpose of fooling the bank to obtain the loan) and the one who KNOWINGLY helps them by "selling their credit line" may be considered an aid in fraud which will have obvious negative ramifications for them. This concern is the biggest risk, not to mention being a/a'd by the credit card company or running into the potential problem of the authorized user gaining access and using "his" card.
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I am not a lawyer, but this seems way too far removed to be implicated.
For starters, when one applies for credit, they can be held accountable only for what they represent (or misrepresent) on the application. No one puts their credit report or score on the application.
It is the grantor of credit that chooses to do their own research, and rely on credit bureaus.
Some will only look at the score, while others might dig deeper into details.
The defaulted creditor might try to implicate the credit bureau, but unlike the rating agencies that gave high ratings to junk mortgage backed securities using totally proprietary and highly questionable formulas, the credit bureaus license FICO, and the rest is just a report of information they have, not an opinion.