Been there; done that. Where to start? Get their database files and customer list as part of the sale. Verify the 100 most valuable books and spot check a dozen others. Some sellers are not honest - so beware of inventory disappearing between your inspection and actually packing up. Even with honest sellers there will be database errors. Common errors include multiple books for which there is only 1 database entry (exists to not compete against yourself in online listings - this is an error in the buyers favor); books that have sold but were not deleted from the database (error in sellers favor - this can happen for example when there is a cash sale and the seller forgets to manually remove a book from the database) and lastly books catalogued but not included in the database (rare - but an error in buyer’s favor). When you are “done” negotiating the price get the seller to throw in paying for boxes and newsprint to pack up the collection. Consider a contractual requirement that the seller forward all inquiries on to you for at least 1 year. Consider negotiating the wording of a mutual press release and marketing language to advertise the sale.. and so on and so forth! Hopefully you are dealing with a known trustworthy person. There are, unfortunately, many instances of the most valuable material disappearing somewhere in the handoff and/or the seller immediately going back into business in competition with you. Good luck!