If the bike is priced well below the book value, it may be a scam. For most shoppers, this is the first turnoff, leading to an immediate dismissal of that bike.
If the seller can only be reached by e-mail, it could well be a scam. Many of these bad listings will have phone numbers posted, but they won't work. Spend the dime and check the number.
On eBay, if the seller offers you a special bro deal and allows you to buy the bike out from under the auction, it's probably a scam. Unless there's a "buy it now" button, this action violates eBay's terms-of-use agreement. Furthermore, it's likely this Joe will try to sell his nonexistent bike more than once.
If the seller demands certain forms of instant electronic payment for all or a large portion of the sale price, it's more likely to be a scam. If the seller is legit, he can wait for more protected forms of payment to be carried out.
Be extremely wary of sellers insisting on their own escrow company. There are many good escrow services, but many others are not on the up-and-up. If the seller demands use of an escrow service, you do the picking; if that's a deal-killer for him, walk away.