While it may appear the the book with a detailed description of a 1 inch dampstain to the lower corner of the last 2 pages is worse than a book listed as being in Good condition, it's likely the other way around. If you're concerned that a book may not really be a first edition as the seller has stated, then ask them how they identified it as a first. If they know, they should be able to answer the question easily and in detail. If you're afraid that the book may not really be in the condition described, then ask the seller to doublecheck and see if anything was missed in the original description. If there is anything at all in the description that you don't understand, ask.
Booksellers often use trade-specific terms.
They are not being deliberately obscure, they are just used to the words, and don't even consider that someone might not understand. If they don't have the time or courtesy to answer your questions, then you shouldn't be buying from them. Look at photos of the item. In an auction, you probably should never bid on items without a photograph. On the databases, photos are much less common.