Generally speaking ebay offers in the main , average quality stamps, but the description if accurate will or should indicate the condition clearly , but ONLY from honest or knowledgeable persons.
That said , price will also indicate condition, if you pay aprox 25% of catalogue value then you should be satisfied with average condition,you get what you pay for in life usually.
Most collectors should know that mint or used stamps that attract catalogue value need to be post office fresh in ALL respects, that is they need to LOOK about a year old ALL OVER if issued within 20 years and LOOK about 5 years old and slightly age toned(cream color) if 50 years plus.
... Hinges on the back are normally ok on mint if over 50 years old , ...that is to most collectors but , some will pay over CV(catalogue value ) for this era if MNH = (mint never hinged ,also known as MUH = british descriptor , mint unhinged, obviously , the USA desciptor MNH is far more accurate and reflects what is desired which is , it never ever had a hinge) .
Note that if extreemely rare then any condition is collectable, one of the worlds dearest stamps has its corners cut off at 45 degrees.
If a child has collected stamps from any era at any time and not paid for stamps from a dealer then USUALLY that collection will be of little value, however a collector that paid a shilling or two for a stamp many many years ago may well have stamps worth $50 each today, but always look closely if you have that oportunity, finds are still made that will buy you a new car.
Hope this is usefull info....last but not least , stamps with edge perforation teeth missing or less than half length teeth will normally be 10% of cat or less and less than that if cut or torn into anywhere, they are called spacefillers, repaired stamps need carefull thought and checking ,but present an opportunity to own a big $ stamp at between 20% to 50% of a good stamp,sometimes a good one is never found in ones lifetime, remember we only are caretakers of stamps, I have stamps with 4 hinges on the back, the past owners would be proud to see them in their sets still ,..or at last?
seller "dukeprince" .... Stam Klector