Checking the Item
When you go to see the item, take a copy of the ad with you to check that it matches the description, and look out for any faults that weren't mentioned. This may provide a reasonable basis to negotiate the price. If the faults aren't acceptable, then don't feel compelled to buy the item.
Finally, and totally unscientifically, remember the "Dog and its Owner" rule. Just as bad dogs and bad owners tend to go together, so do bad sellers and bad items. If you don't trust the seller, don't buy the item.
Depending on the item and its value, it may be worth having it professionally examined before you buy, either through a professional body or other knowledgeable source. They may examine the item and be able to give you more safety with your transaction.