I have purchased lots of bike parts/flyrods/yarn from eBay. My husband is getting some gorgeous hardwood blanks. Other than brand new shoes, I have never purchased clothing. So far, we've only had good experience but we only buy items that we've researched and have good pictures. We also look carefully at the seller's comments. I also only buy in US/Canada just so if I do have a bad experience I might have a better chance of dealing with the seller. The number of times bike components sell for more than you can get from any of the big name bike catalogs is amazing, especially when realize that these companies generally give a better satisfaction/return guarantee. Some of the catalog companies also sell their stuff on eBay so keep an eye out. You can also find a lot of stuff that hasn't been made in a while (like a new Suntour freewheel for your cherished vintage bike).

So, yes, eBay can be a good thing, but you have to know your product. Email the seller with any questions. Their response is critical as it indicates their actual knowledge of the product and the timeliness of their delivery.