A check/cheque is a written document authorising your bank to transfer a sum of money from your account to a named person, either as a deposit or as cash.
A deposit is a transfer of money made from your account into another person's account.
The illegality in this passage is that the "purchaser" sends more money than is actually required, then asks for the difference to be returned to them, but in fact has never made any real transfer of moneys as there are none. At no time has he any intention of in fact making a purchase. He is cheating you of the money you are being asked to "return".