If you sell anything online, you can potentially be the victim of a scam.  It doesn't matter if the site is Horsetopia or Craig's List.  

You, as the seller, must educate yourself on what to look for so that you can avoid becoming a victim.

Example:  I am selling a horse for $2500.00.  I am contacted by a potential buyer who is going to give me $2500.00 plus an extra $1500.00 for the shipping agent.  The potential buyer wants me to provide cash or a cashier's check to the shipping agent.  The buyer sends me a check for $4000.00. Sometimes it may be drawn on a personal or business account or it may even be a cashier's check that is sent via, fed ex.

Many times these checks may look legit, but often times there may be issues with the printing, such as with the numbers containing the routing and account number.  These numbers contain a magnetic ink so that they can be read by other banking or financial institutions.  With current printers, sometimes it is difficult to tell the discrepancies.    

What you can do: 

• Take the check to your bank and have them look at it and do some research. 
• You can google the routing number and find out the bank name, phone number and address. 
• You can call the bank the check was purportedly drawn on. 
• If you want to deposit the check and see what happens, do not put it in an account you use frequently, and hold the check for at least 30 days to see if it clears.  Remember that this is not always a guarantee that the check won't come back fraudulent. 
• If you use the funds from the check and it comes back fraudulent, you may be responsible for any and all transactions. 
• This will include checks that you wrote that were returned as insufficient, because of overdrafts. 

If you have a crime involving the internet;

Filing a Complaint with the IC3 - www.ic3.gov