It really depends on who is selling it. If it's someone who is tech-savvy and uses eBay and the likes, tread carefully as they may have researched the potential selling prices. If not then make an offer that would seem a reasonable amount to someone wanting to get rid of an old computer. But at the same time don't offer anything cheekily low as they may tell you to get lost!

I'm no expert on the American Apple market but Apple //c computers in the UK ususally make between £50 and £100 on eBay and that's without the box. Admittedly they are far rarer here than in the States.