contango 

Without seeing the ad, pictures etc it's hard to come to a definitive conclusion.

On one hand the guy picked up the bike in person. If I was buying something listed as needing some attention I'd want to check it over to make sure it had been described sufficiently accurately that I knew what I was getting into, and if someone does collect in person it's reasonable to expect that they will notice flaws that are clearly visible. Likewise if the pictures were of sufficient detail and quality that the broken spoke was visible and could reasonably be expected to be seen then I'd say you've got a good case to say it was shown in the pictures even if not explicitly mentioned in the text.

On the other hand if you said the wheels where fine (or made statements that reasonably led the buyer to believe they were fine) and then handed over a bike with a problem with the wheels, I'd say the buyer has a case to either reject the bike as being not as described or make their own suggestions of what to do about the situation. I would be inclined to say the buyer might have a case to ask for a replacement spoke, the only question is where to draw the line between returning the bike to the condition as advertised and giving the buyer something significantly better than what was advertised.

It may be that he's trying his luck hoping to get a bike that's been unused for years and has known issues at a price consistent with that, but then getting you to pay to make sure some part of it is pristine; it may be he genuinely wouldn't have bought it if he'd known it had a broken spoke (personally I find that hard to believe, but you never know). If you want to flush him out if he is just trying his chances why not just say (assuming it's true - again I don't know what the listing said) that the bike was advertised as needing attention, it needs attention, and you're not going to pay for the bike to be repaired and if he doesn't like it then return it for a full refund. If he's truly dissatisfied then he'll return it and you can then do with it as you will; if he's trying his chances hoping for a freebie he'll probably shut up. Either way if you've made it clear you're willing to give a full refund if he's not satisfied with it I'd have thought you'd be in the clear as far as ebay is concerned (I've never sold on ebay, so can't comment from a position of experience)

If he does try to return it then, from what you've said, it would seem like a pretty clear case that you sold something you hadn't used in a while, forgot to mention one defect among others you did mention, but then refunded the money of the guy who bought it not noticing the extra defect. I'd say the offer of a full refund was an entirely reasonable option.