To illustrate what I have said about how wise collectors can use the ebay marketplace to great advantage, I will present info on 7 covers I purchased from ebay auctions this past year. With regard to vintage, condition, and cost, these covers are representative of the thousands I have purchased from ebay auctions. Please understand that I could have listed 70 or 700 similar covers.
Before I get to the covers, I want to note that I enjoy very positive relationships with hundreds of ebay sellers. These are sellers who have proven their trustworthiness to me. Their merchandise has rarely disappointed. They value me as a customer because of my high volume of purchases and my rapid payment and because I only return items for refund when there is cause. Yes, there are a lot of very mediocre sellers on ebay, and some outright crooks, but one learns not to do repeat business with them. Experience is a great teacher, yes?
All of the covers that follow were solid buys, in my opinion. Some were outstanding. I'll rate each.

This is the Bahamas high value 3 shilling stamp issued in 1930, the one with dates. It is Scott #89. The 2007 Scott Specialized Catalogue gives a postally used off cover value of $110. I purchased it for $38.75 (all purchase prices include shipping cost), 35% of CV. Considering condition and scarcity, I believe this is a very good buy.

This is the France Rouen Aviation commemorative cover issued on 9/2/1922. I paid $10.55 for it. If you are familiar with the prices this cover usually fetches, you know this is an outstanding buy. I got it cheap because the seller did not identify it properly.

This is the complete Berlin/Goethe set on card. The 2006 Scott lists a postally used off cover value of $182.50 for the set. I paid $21.50 for this item, 12% of CV. Why did it come so inexpensively? The seller did not know what she was selling. In fact, her scan showed the front of this postcard, which is meaningless. It slipped through the cracks into my eager hands. A fabulous buy.

This is the one shilling stamp Great Britain issued in 1856, Scott #28. The CDS is 03/02/1861. This cover is in superb condition, a gem. The Scott 2007 Specialized gives a value of $425 for this cover. I paid $47.25, 11% of CV. A very solid buy.

This is a very early feeder flight cover from Tanganyika. Very fine condition. The feeder service covers are somewhat scarce and in high demand. I paid $11.35. I believe true market value is in the $29 - $35 range, based on other ebay sales. Not a spectacular buy, but a cover with some great history for a very reasonable price. I treasure it as much as the Berlin/Goethe item.

This is NZ Airmail Catalogue #83r, Blenheim to Invercargill dated 12/24/1932. It is signed and backstamped with NZ Airmail Society authentication. The catalogue states there were only 60 covers on this flight. It gives a value of $65. I paid $34.58. Not an earthshaking acquisition, but a rare item in superb condition for a very reasonable price, I believe. I'm proud to own it.

These are the first four airmail stamps issued by Saar on one very attractive cover in superb condition. An infrequently seen item. The selvage adds to the value. The Scott 2007 Specialized lists on cover values of $12.50, $21.00, $17.50, and $175 for the four stamps. I paid $59.00. Not a spectacular buy perhaps, but quite solid on the whole.
My approach to ebay is labor intensive. When I am in high purchase mode, I scan every vintage cover listed on ebay that day to try to find the ones likely to fall through the cracks. You might not be willing to be as patient and thorough as I am. But if you are, and if you study bidding patterns long enough to develop a firm grasp of market value, you'll be very pleased with your acquisitions.
I would like you to accept that I know what I'm talking about.