You can freely sell posters, cards, clothes or other physical items which have got printed on them an image taken from Unsplash or Pexels collection. The image cannot be printed as is, it needs significant manipulation, says Unsplash. Pexels says that the image must be modified. You wonder what this means?
A guess: It's manipulated or modified enough if the original image cannot be separated i.e. a substantial part of the image is removed, mangled unrecognizable, distorted or covered.
But no generally valid measure exists. And the guess cannot be a legal advice, because it's only a guess. Unsplash and Pexels have left to themselves the freedom to decide every case separately. It maybe is modified enough if Unsplash nor Pexels do not sue you. The only 3rd party authority who has power to say which is enough without asking from anyone is the court of law after you are sued.
Before a legal attack against you Unsplash nor Pexels have no duty to tell anything to you. Fortunately they have published their licenses which allow derivative works to some (not exactly defined) degree.
To be sure no troubles will arise use your own images. Or try to get a written clearance from Unsplash or Pexels that you have modified the image enough. Know that nothing forces them to answer anything and they still can sue you if they want.
If you suspect your manipulation is too weak, it very likely is.
You are not allowed to distribute Unsplash nor Pexels images as files. That means selling them, giving them for free and letting them be available for downloading are forbidden. Any modification does not change that. As long as someone by making side by side comparisons or scientific analyses could see that you have created your image by mangling Unsplash or Pexels image you have made a copyright infringement.