Upload & Sell: On
While you might be in RGB 8bit mode, you may still be working in a different color space.
Look under Edit down to find Assign Profile and Convert to Profile. Open Assign Profile and you can see what your current color space is that you are working in. Using Assign Profile you can choose to work in a different space. However, you can also use it to preview what it will look like to others if they are looking at it in a different space. The point here is to readily see how different it can look to others on the web where sRGB space is most prominent used/assumed.
If you are working in a wider space like Adobe RGB, ProPhoto RGB, or CIE RGB, assigning to the smaller sRGB space will look differently, often times muted by comparison to the wider spaces. Just because you are in RGB mode, doesn't mean that all RGB spaces are going to look the same if others are viewing in sRGB while you have been working in Adobe RGB or ProPhoto RGB.
For this, you may want to use Convert to Profile (as Peter mentions) for posting your files to the web. When you do convert spaces, PS will try to emulate/match the image so you may not notice a radical change (like when you changed the space using Assign Profile), but if you have converted it to sRGB (and imbedded), it will have a better chance of being viewed by others as you have created it.