"If the pictures are in the public domain, IE: clipart, they might not have needed releases in their thinking. MLB might think differently"

Most such public domain images still carry copyright or 'unauthorised use' caveats. Placing the image on something that could be construed to cost $5 could breach this.

There was an ebay chatboard thread on this before. Possibly mentioned here also. It discussed the 'big names' that review every item appearing on ebay with their name on and the fact they will go after anyone to protect their image.

I seem to remember someone posting they had ebay items ended merely when they used some company name just for comparison.