I am being taken advantage by a guy on the internet he is trying to give me less money than he promised for silver I already sent him

Unfortunately, without some written agreement, there may not be much that can be done. If you sold something to someone on an online auction or through an online vendor, you may be able to do something through contacting that particular site. Not to sound negative, but I get the feeling that you don't know this person or have a written agreement with them? I wish you luck.