There are actually 4 major markets in the e-commerce arena, they are B2B, B2C, C2C and C2B.
B2B is a business selling to another business. The best example I can think is I buy items from a warehouse to resell. This is a B2B transaction, as the wholesaler will not sell to consumers, only businesses that resell the merchandise. However, as pointed out earlier, not all B2B transactions invovle merchandise that is to be resold. You can get a business account with a business that sells stationery, computer, office equipment and many other products. The most common use for this is bulk buying (offices generally need more than a few packs of paper from wal-mart or other retailer), specialized items, or just getting better deals than you would as a general consumer. Many B2B sites require registration, credit verification and a copy of a state sales tax license or other form of a sellers permit or business license. This is to ensure that you are an actual business entity and is generally used even if you do not plan to resell the merchandise that you purchase. Many times they will also offer net terms or other credit terms rather than requiring payment up front. Net terms refers to when you order something you don't pay up front, instead you get an invoice and it is generally due in full in 30 days, though you can have terms such as 20 days or 45 days. This is generally noted as Net 30-days. Most consumer oriented businesses will not do this for the end user purchaser.
B2C would be when you buy something from walmart.com, they are they business, you are the consumer buying the product at the final retail price from a retailer. Generally you pay up front for the merchandise (cash, credit card, check) unless they offer financing. Even if you finance something you are still in a B2C envrionment.
C2C would be a site like eBay. Consumers selling to consumers (yes, eBay does have quite a few businesses that sell on there). When you buy an item from another individual (not a business) that is a C2C transaction.
C2B is found commonly on eBay or other auction type of sites. A business buys from the end consumer. An example would be I have a really cool computer I bought, I don't want it any more, I sell it on eBay and a business that wants it buys it from me, the consumer.
An example of a B2B site would be
www.dandh.com (computer distributor). They require copies of business license/resellers permit and an account to be created just to access pages of their site other than just the home page.
An example of a B2C site would be
www.walmart.com or
www.target.com, they sell to anyone, regardless to whether or not you have a resellers permit or sales tax license.
And of course
www.ebay.com as the best C2C example.
The basic analogy is this:
B2B = Business use of merchandise bought from a business
B2C = Personal(consumer) use of merchandise bought bought from a business
C2C = Personal(consumer) use of merchandise bought from an individual (consumer) entity.
C2B = Business use of merchandise bought from an individual (consumer)