In order to make the system run smoothly, the secondary market participants set out guidelines for primary lenders. If the primary lender conforms (follows the guidelines in making the loan), it knows that the loan can be sold. Since a large percentage of primary lenders make loans with the intention of selling them, most lenders only make loans that conform to the requirements of the secondary market.
The sale of your loan does not affect your interest rate. The secondary lender buys loans based on prevailing interest rates. If a retailer wants to sell the loan, it knows the interest rate that it must charge to sell at face value. If a higher rate is charged, the loan may be sold at a premium, an amount higher than face value. If lower than the prevailing rate is charged, the loan will be sold at a discount, meaning less than face value.