Well I wouldn't spend more then $50 for the Mona Lisa painting.. but that doesn't mean it is worth that much.
As far as haggling goes, just ask the seller what range he is looking for and go from there. Generally if you are selling something I expect you to give me a price estimate to see if we are in the same ball park or not (sellers do have a price in mind when they decide to ditch something, reasonable or not). If I see something somebody has and want it (not advertised as for sale), then it is up to me to give them a price to see if they are interested since they havn't had time to think about pricing.
Anybody who knows anything about computer collecting knows that a non upgraded Lisa 1 is pretty much destined for ebay and not some low balling local buyer. Outside of forced evictions or having to move quickly because of unforseen work requirements you tend to not find rarities for cheap anymore (stuff where the seller doesn't have time to ebay it).