The definition of crazing is: To produce a network of fine cracks in the surface or glaze of, AND: To become covered with fine cracks. FINE to me, and then to you, can be two altogether different things. That is why as sellers, we must describe things as detailed and honestly as possible, and photograph pieces as accurately as possible. As buyers, we must educate ourselves.
CRAZING on pottery... it happens mostly with age, sometimes with temperature and humidity change (like if you move your pottery collection from Arizona to Maine.) Crazing also happens if the piece was used as a planter from water moisture damage, which is really to be expected in most pottery planters-- that's why they were made, to hold a plant. Crazing can also happen if the piece gets vibrated often (like from being in storage, or moved around a lot). The most common reason is plain old age though, just like us imperfect humans and our inevitable wrinkles, so most experienced collecting folks don't mind SOME crazing. In selling, it is always best to describe crazing as detailed as you can. (Such as: deep extensive crazing all over piece but no cracks, chips or nicks... OR... very slight crazing on handle and rim, and inside bottom, but none on exterior... etc.) The more you can tell people before they bid, the better.
If you are bidding, ask questions. My motto on eBay has always been this: IF THEY DO NOT REPLY, DO NOT BUY. An honest seller should be proud and glad to share any information about their listings. I always try to be as descriptive as possible with any flaw I see, and many times folks will say in their feedback to me that the item was much nicer than described.
Now, CRACKS are a different story in my opinon-- I personally am EXTREMELY careful buying anything online with even a hairline crack, since a hairline can expand in shipping, and one man's hairline crack is another's earthquake faultline! I have, however, sold some VERY old (1930's) pieces of American pottery that were extremely crazed, had light cracks from a plant's moisture, and even a chip on the bottom of one. The reason I sold them, and had happy customers, was that I described EACH & EVERY FLAW in GREAT DETAIL, so there would be no mistaking that I had listed the items honestly. These particular pieces were so old & rare that the supposed flaws just didn't matter at all to the buyers. HAIRLINE CRACKING easily occurs in planters, again because of moisture, also if a plant becomes root-bound it can actually change the structure of the piece.
If you are trying to sell vintage pottery pieces that have crazing or hairline cracks, try to photograph them in the light so these things can be seen, and if you can't get them to show in the photos, just say something like: CRAZING/CRACK NOT VISIBLE IN PHOTO in your listing, but be detailed in your description. As far as cracks and crazing effecting worth and pricing-- just like anything on eBay, an item is worth exactly what someone pays for it THAT MOMENT. The next day, the same item may sell higher or lower... and that is the new worth.