The purchaser creates [forges] a laser printed check in the name of a business whose name, address, they have obtained along with the actual account and routing number of that same business.
That forged check is sent to a transportation company requesting that they deposit the check and purchase and/or ship goods for the party that has contacted them.
Or, the forged check is sent to a seller [listing a car in the newspaper or craigslist] that is for more than the agreed upon price of the price being asked. With the check will be instructions or previously texted or emailed instructions requesting that the seller deposit the check and then return the difference to the buyer via Western Union or to use the difference to pay the transportation company when the carrier picks up the goods.
What the fraudulent issuer is hoping the seller will do, is quickly deposit the check and Western Union the difference and/or immediately contact the carrier to pick up the goods.
If the recipient attempts to cash the check, the bank will usually call the account holder of record to verify this proper issuance of the check. But if the recipient just deposits the check into their account their account may initially be credited with the deposit while the check is being processed against the account of the victim of the forgery. If the check exceeds the funds available in the victims account then the victim is notified and the the funds in the depositors account are held or debited until the check clears. The hope of the fraudulent actor is that the check initially clears and that the fraudulent check is not brought to the attention of the bank until after the initial recipient of the check has sent the over-payment and/or goods to the fraudulent issuer.
To prevent this fraud, all the check recipient needs to do is contact the business or bank listed on the check.