minus the 2.5% fee directly from the selling price fee = price * 0.025 earning price = price * 0.975.

fixed desire selling price, add more to enable 2.5% fee taken by OpenSea earning = price fee = price / 0.975 * 0.025.

we use fixed salt for selling order from mob contract since the signature is not related to salt value, using fixed value is helpful to avoid re-submit the same listing.

hex for "0x42" text string.