Seller credits are treated as a reduction in selling price.
There are several other selling expenses that also reduce your capital gains, see publication 523 starting on page 8.
https://www.irs.gov/forms-pubs/about-publication-523
However, if you received a 1099-S for the full listed selling price, the IRS may send you an inquiry letter if the sales proceeds figure you report on your tax return does not match the 1099-S. In that case, you can chose to enter your selling expenses as adjustments to the cost basis instead. In other words, suppose your purchase price was $150,000, the selling price was $300,000, and you paid an $18,000 real estate commission and gave a $10,000 sellers credit to the buyer. Your net sales price is $272,000. However, if you received a 1099-S for $300,000, you might choose to report the selling price as $300,000 and the purchase price as $178,000. The net capital gain is the same, but reporting the 1099-S figure as the selling price may reduce your chances of getting a letter from the IRS asking to explain the difference.
