If the chain has a special link that lets you take it off easily, give it a good scrub with some washing up liquid and a very stiff brush. That will get the worst of the surface rust off. If it's a low end bike I doubt it will have a chain that lets you do that sort of thing.

So your options are to either try and scrub the chain while it's on the bike, or buy the cheapest suitable chain you can find, or let the buyer deal with the fact it's a used low end bike and it's not going to be pristine. If you were buying a bike for $4000 you'd expect it to look nice when you went to see it, you'd want to see the seller had looked after it, and that it didn't look like it had been left to rot in a dark corner. If you were buying a bike for $50 you'd naturally have much lower expectations.

If, as bobotech mentioned, you'd optimistically hope to get $100 for the bike then you get to decide whether to pay a few bucks so you can truthfully point out that the chain is brand new, or you can slap some lubricant on it and show the buyer how smoothly the train runs and that they don't need to worry about a bit of surface rust.

If you replace the chain be aware that if the cassette/freewheel is worn you might find the chain skips (this would depend on how worn they both are), so for a cheap bike you might be better off just scrubbing the chain as best you can to get the worst of it off, oiling it, and hoping your buyer isn't expecting $4000 quality in a $100 bike.