Your number one resource when considering making a purchase online is to ask questions. If the item is glass, porcelain, or other easily breakable material, inquire upfront about cracks or chips. Ask if the rims are intact, or if there is any crazing. Ask the seller if they have thoroughly inspected the merchandise, and if not, who did, and why it was not them. If the item is of a material that could be damaged by being in a household where there are smokers, inquire if the item has been stored in any location where people smoke. If water damage is a possibility, ask if the item has been stored where any damp could get at it. For books, ask the seller if there is any spotting, commonly referred to as foxing, which is a sign that the paper is damaged from dirt or age. If a photograph the seller is providing does not show an area of interest clearly, ask for a better shot. In this modern age of email and digital cameras, an extra photo or two is only going to take the seller a few minutes to accomplish. If they begrudge you asking, you most likely do not want to purchase the item in the first place.
Do not rely solely on photographs though, as a defect may not show clearly, such as staining, so ask upfront. If the seller has noted any special circumstances about the item that are not clearly defined in a photograph that would affect a purchaser's decision, they will often be honest in their answers if asked directly.
Paper trails and Record Keeping
While you cannot always trust a seller to be truthful, places such as Ebay will often back up a purchase if correspondence between seller and buyer is provided to back up claims. Keep a log of any communication that is related to a deal until the item has arrived and is determined to be in the condition it was expected. This is not a guarantee that a deal that goes bad will be resolved satisfactorily, but it can only be a help to your case.