How much can you sell an old school Redline BMX for?

It depends on how old school it is. If you have an old 70's Squareback Redline or the sort, it could be worth quite a bit depending on the condition it's in. Other individual models are valuable as well, but most aren't worth even half of what they cost new. It depends on the bike.

Hit eBay and Craigslist to find out what similar bikes are selling for.