From what I can see, this topic has been discussed as long as there has been engines, but why not ask again! To my defence, this engine is not built to upgrade HP, just stock, and most disussions (all?) seem to be about how to make an tuned up engine run safely at high RPM:s.

So here it goes, please be gentle. Is it necessary to balance the rotating assembly when replacing pistons with aftermarket? I don't know yet which pistons have been put in, but the seller of the engine suggests it's "stock" but not OEM. Someone once said that when using all OEM (stock?) parts, it doesn't necessarily have to be balanced since it's not individually balanced from the factory and there's higher tolerances in that kind of setup.
But with aftermarket pistons/rods/etc., the weight of the components will be different from OEM/stock and balancing will be necessary, otherwise it'll be excessive wear the main bearings/crank.

I'm looking to buy a rebuilt long block, which I'd rather not tare apart first thing. Seller don't know if it's been balanced, and I'm just assuming it hasn't. It's within my strained budget, and an OEM reman longblock is not an option at this point. I'm having a really hard time deciding if I should walk away or if I can live the uncertainty if it's been balanced or not.

It'll be running in a 30ft cruiser at about 3000-3500 rpm or below 99% of the time.

If the weight of the new pistons is significantly different than the oem ones then you definitely want to rebalance. If they are within 10 grams then you should be ok, that is well within factory tolernaces. Much more than that and I would balance it.

Your seller says it was "rebuilt" ? What does that mean ? Does he know who did it ? Has it been run since it was "rebuilt" ?? If the oil pan is off you can usually tell if its been balanced or not by looking at the drill holes in the counterweights. If the pistons used were direct replacements for the oem ones then they are usually close enough buts its always a good idea to check.

I’d walk and wait for one to become available that has history on parts and machine work.

Being balanced is the least of the things I would be worried about. Many a buyer has been ripped off by a "rebuilt " engine since their is no industry standard. A rebuild to some is a can of spray paint and a rebuild isn't a rebuild to others unless it has new rods and crank.
You could tear it apart, measure everything and either accept it or leave the seller with a box of bad parts.

Thanks for the answers, I skipped it and found a complete, running 170 hour 496 HO 2004 for the same price. Just have to be patient