Is it possible to sell photos to adobe stock even though I've used that very photo on my Instagram account?
Yes. And no. That is, you do NOT sell photos to Adobe stock. You ,license them, and they pay zero. You get money only if one of Adobe's customers pays them to license your photo. But you do not have to give Adobe exclusive rights, so yes you can post them on Instagram before or after. Instagram hardly matters, anyway, they don't give access to the high resolution original needed by advertisers.
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The images you have uploaded to your Contributor Account on Adobe Stock are non-exclusive; you may post them to other stock agencies, post them on social media or photo sharing sites, or sell them yourself as often as you desire. You still own the image.