The threshold question is whether the seller is a "creditor," and for most sellers the answer is no. Regulation Z at 12 CFR 1026.2(a)(17)(v) says a person "regularly extends consumer credit only if it extended credit (other than credit subject to the requirements of § 1026.32) more than 25 times (or more than 5 times for transactions secured by a dwelling) in the preceding calendar year." A homeowner selling one house and carrying the note is nowhere near five dwelling-secured extensions, so the bulk of Regulation Z's creditor obligations, including the ability-to-repay rules written for creditors, do not attach.
The same sentence contains a trigger nobody expects, and it works on a count of one. It continues: "A person regularly extends consumer credit if, in any 12-month period, the person originates more than one credit extension that is subject to the requirements of § 1026.32 or one or more such credit extensions through a mortgage broker." Section 1026.32 is the high-cost mortgage rule. So two high-cost seller-financed deals in twelve months, or a single one arranged through a broker, makes the seller a creditor with no volume test at all.
The loan-originator exclusions are two different tests, and the difference is the balloon. A seller financer who meets 12 CFR 1026.36(a)(4) or (a)(5) is not a loan originator. Paragraph (a)(4) is open to any "person," including a company, covers the sale of three or fewer properties in any 12-month period, and requires that the financing be "fully amortizing," that the seller make a good-faith determination that the buyer has a reasonable ability to repay, and that the rate be fixed or adjustable only after five or more years with reasonable annual and lifetime caps against a widely available index. Paragraph (a)(5) is open only to "a natural person, estate, or trust," covers one property in any 12-month period, and requires only "a repayment schedule that does not result in negative amortization," plus the same rate condition.
Read those two lists side by side and one practical rule falls out. A balloon payment is not fully amortizing, so it fails (a)(4); but a balloon does not cause the balance to grow, so it satisfies (a)(5). An individual selling one house may carry a note with a balloon and stay outside the loan-originator definition. A company selling three cannot. Both exclusions bar a seller who built the house in the ordinary course of business.
A seller who still owes a mortgage is making a transfer the lender can act on. The federal rule preempting state due-on-sale laws defines the trigger broadly. Under 12 CFR 191.2(b), a sale or transfer "means the conveyance of real property of any right, title or interest therein, whether legal or equitable, whether voluntary or involuntary, by outright sale, deed, installment sale contract, land contract, contract for deed, leasehold interest with a term greater than three years, lease-option contract or any other method of conveyance of real property interests." The protected transfers at 12 CFR 191.5(b)(1), which cover things like a transfer on the death of a joint tenant, a divorce decree or a move into a living trust, do not include a sale to a buyer. So the seller's own lender may call the loan due. The mechanics of that clause and the transfers Congress carved out are covered on the assumable mortgage page.
The interest rate is not entirely the parties' to choose. Where a deferred-payment sale charges no interest or too little of it, Internal Revenue Code section 483 treats part of each payment as interest anyway. It defines "total unstated interest" by discounting the deferred payments at "the applicable Federal rate determined under section 1274(d)," which means a seller cannot convert interest income into sale price simply by writing a low rate into the note. The rules apply to payments due more than six months after the sale under a contract with payments due more than a year out.