for first time sellers, use escrow. That should be the default for pretty much anything on the forums, really. Check reputation if they're relatively new and just getting established. Be wary of 'too good to be true' deals.

I sell Amazon gift codes/will buy off Amazon wishlists at a bit below bitcoin market price. I usually do 3% off because I get 3% back on Amazon purchases through my credit card. I've had the card and Amazon account for many years, been selling gift codes on here for around a year maybe...

That said there are some seemingly legit sellers on here who sell for ~10-20% off regularly - no idea if they're just willing to pay that much more for BTC or if they really have some nice deal worked out somehow, or if they are doing something illegal someone along the way... Anything more than 20% off I'd assume is a scam/stolen credit card buy.