My mother bought some sneakers but they weren't the right size from eBay. What can she do?

If they were a different size than advertised - she can return them.

If they were the size she purchased - then nothing. 

Contact the seller and ask if they accept returns.

She can't figure it out on her own?  If you are going to help your mother with this very basic problem, you ought to know that eBay does NOT sell anything.  They are a middle man.  That's all.  Since the mistake was hers, it is unlikely that the seller would accept them back.  Tell her to list them on craigs list or to donate them.

Possibly return them or possibly sell them to someone else.

Try making her feet smaller