To put it bluntly, they're most likely trying to bend you over.
ALL engines have "blowby" because there is no possible way to completely seal off combustion pressures from the crankcase past the piston rings. Even "gapless" or "total seal" piston rings aren't 100% leakproof.
If you have a steady stream of "blowby" when the oil fill cap is removed, the engine is fine (as long as it's not enough to force the cap off of the oil fill tube with a vengeance). Excessive cylinder pressure escaping into the crankcase from one (or a few) cylinder will cause the vapors from the oil fill cap to pulse (you'll get defined "chugs" of vapor coming out). That's bad.
If a salesman couldn't explain it, he probably heard it from one of his supervisors. In this case, he's using your inexperience to steal the truck from you....literally. Excessive or rhythmic pulses from the oil fill tube are bad, but if your truck runs fine (hot and cold), I suggest selling privately because ANY dealership is going to buy at wholesale and resell at retail. Anything a salesperson can do to make more money on trade-in (regardless if he even knows what he's even looking for), he's going to do it. Car dealerships don't make much money on new car sales; used vehicles are cash cows.
Your BEST option is to sell the truck privately, pay off what you owe, and then take the remainder for a down payment on a new vehicle. You'll get the most out of the truck you have now that way.
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