I give this a 99.999% chance of being Brachyplatystoma capapretum (which is called the 'false piraiba'; it ONLY gets to 4' long). They're profoundly more common than true piraibas, cheaper than true piraibas and unless you've spent a LOT of time investigating the difference, nearly impossible to tell apart. The price for this one is is the ballpark for typical capapretum prices as well. The half dozen true piraibas that have come in over the past decade have all been well over $1000 and still sold instantly.
We need pics.