How piggybacking works and why it is legal
When you are added as an authorized user, most issuers report the card's full history, age, limit, and payment record to your credit file. A thin file inherits an aged, high-limit, perfectly-paid tradeline overnight, and scoring models reward it.
The legality rests on the Equal Credit Opportunity Act. Regulation B's treatment of authorized user accounts is why spousal piggybacking exists, and why the bureaus and FICO cannot simply ignore AU tradelines without harming legitimate users. The expected CFPB rulemaking that might have narrowed this was withdrawn in May 2025. Adding an AU, including a stranger, is not illegal.
The broker market
An industry of tradeline brokers matches cardholders with old, high-limit cards to buyers who want the history. A slot on a $20,000-limit card with five-plus years of age sells for roughly $200 to $1,500, with the cardholder taking a cut for each 60-to-90-day rental. The AU never gets a physical card or charging ability; they are buying the reporting, nothing else.
What it actually moves: thin and young files can jump 20 to 100 points under FICO 8. Thick files with their own derogatory history move much less, because an AU line cannot erase a charge-off. Brokers quote the thin-file numbers to everyone.
Why the asset is decaying
FICO 8, the score most lenders still use, counts AU tradelines with some anti-abuse logic already. FICO 10 and 10T go further and discount tradelines that do not look like genuine household relationships. As mortgage scoring migrates to 10T over the next few years, the value of a purchased stranger tradeline shrinks toward zero exactly where it matters most.
Anyone selling tradelines is selling an asset with a visible expiration arc. The trade still works today on FICO 8 underwriting, and it works less every year. Price that decay into any decision, because the broker will not.
The legal line
Buying a tradeline is legal. What you do with the resulting score can stop being legal fast. A loan application asserts your creditworthiness to a federally insured institution; using a purchased score boost to misrepresent who you are as a borrower, on a mortgage application especially, edges into bank fraud territory. The cases that have been prosecuted in this space involve exactly that pattern: paid tradelines plus material misrepresentation on applications.
The version with no legal exposure and no broker fee is the original one: a family member with a long, clean card adds you as an AU. Same mechanic, real relationship, no misrepresentation, free. For young people building a first file, a parent's oldest card remains the single highest-value move in all of credit building.
Do not touch: primary tradelines and CPN bundles
Adjacent to the AU market sits an openly criminal one. Primary tradelines for sale claim to add you as the original account holder on an account with history, which is not a thing any legitimate issuer does; the products are fabricated accounts or identity fraud, full stop.
CPN bundles are worse. A credit privacy number sold as a fresh credit identity is, in nearly every case, a stolen Social Security number, often a child's or a deceased person's. Using one on a credit application is identity theft and bank fraud with penalties measured in decades, and the market for them is growing, which means enforcement attention is too. No score is worth it, and this site will never describe a method that touches them.
It is worth noting what the expensive courses in this niche tend to do with this topic: gesture at seasoned tradelines as a secret of the pros, skip the FICO 10T decay, and skip the legal line entirely. The unbranded truth fits in this one page.