You have on both pictures noticeable noise. The "nails" picture is also not sharp enough (extract at 100%).

Also, the "shell" picture is missing those fine details that are necessary for stock use (extract at 200%, the noise is eating the details).

You really need to look at your pictures at 100% and/or 200% to see defects that disqualify the asset for stock use.
ISO 800 is a lot for this type of camera, you should go as low as possible with ISO to get less noise. Use a tripod to get a stable camera, use the f that gives you the best possible sharpness for your lens (each lens has a sweet point). Get the exposure time correct for the situation.
Apart from the tripod and the ISO recommendation, you can, of course, adjust your camera as you see best. However, the result must be as free of defects as possible after processing.
If you are new to stock, you should consider these resources: https://helpx.adobe.com/stock/contributor/tutorials.html
Please read the contributor user manual for more information on Adobe stock contributions: https://helpx.adobe.com/stock/contributor/user-guide.html
See here for rejection reasons: https://helpx.adobe.com/stock/contributor/help/reasons-for-content-rejection.html
and especially quality and technical issues: https://helpx.adobe.com/stock/contributor/help/quality-and-technical-issues.html