I hate it for the OP.
I've bought and sold stuff on truck and car forums for years and never had a problem. What I do:
1. Contact the person directly through the forum. If someone on here advertises they have a bumper for sale, I send them a PM and make first contact that way.
2. Once I've made contact with the person - I can see the same person advertising has PM'd me back - I ask for a direct cell phone number, which I will then text and have some conversation with them to be sure the number is real and related to the sale. I also get a real name and address.
3. I send money only using some kind of credit card so it can be disputed for fraud if necessary. Paypal, Stripe, Square, something like that. If they don't have any kind of electronic payment portal that takes credit card, then no dice. If I have to pay the CC fee so be it.
4. I call them and speak to a real live person before sending the money. You can pretty quick figure out if something's legit or not that way.
To date, I've never actually been scammed. I've bought a part or two that wasn't quite what was represented, but always got something back for my money. On balance it's worked out pretty well - most people are honest and if you just take some basic precautions your chances of being scammed goes way, way down.