The buy rate is the funder's base price for your deal, as quoted to the broker. When the broker's commission is built into your price, the rate you are offered, sometimes called the sell rate, sits above the buy rate, and the difference raises your payback.
Example: A funder quotes a buy rate of 1.28 on $30,000. If you are offered 1.34, the extra 0.06 adds $1,800 to your payback.
Not every deal works this way. Some funders pay brokers a set commission at their standard price. You rarely see the buy rate itself, so compare the total payback and the estimated APR across offers from different sources.
Ask the broker directly how it is paid and whether its compensation affects your rate. Openbook Capital tells you how it is paid on your deal.