Here's a problem: broad matches on keywords that have two meanings (homonyms - for my second grade English teacher....see I did pay attention!). Words like plant, resume, wind, record, produce, etc... And here's another one: other variations of the services you need to negate. Auctions are a great example for that one. So what is the problem with these? Let's go over them.
Homonyms
When you use a general word on broad match that has a second meaning, half of the time you could be showing up for non relevant queries.
Not only do the lack of clicks in that case lower your quality score and increase your cost per click, but you may end up with non relevant clicks costing you money.
One thing we like to do with these is to take the non relevant version of the word and do some two and three word searches in the Google keyword tool to generate a great list of negative terms to include.
It is important to remember to negate the other meanings of these words to keep your costs down and your quality score high so that you get more exposure in the ad bidding auctions.
Services with other variations
When you have auction sites, etc... you forget that there are a ton of different ways that auctions can be run, as well as types of auctions. You have silent, blind, penny, fixed, airport, online, car, estate, auction houses, high end, low end, etc...
If your business only focuses on one kind of auction, but you insist on bidding on general terms like auctions, you could be wasting a ton of money and relying on your targeted terms to bring in a reasonable conversion rate.
Auction sites usually use PPC as a customer acquisition tools instead of turning a profit off of a single sale like many online stores and merchants do. This is because auction sites usually have long term or repeat customer relationships and purchases with multiple bids and sales which generate multiple revenues each time. The addiction to bidding generates a lot of income which is why they have a larger margin for customer acquisition.
If you are running PPC for services like auctions, remember to include things like airport codes, cattle, sheep, cars, clothing, christies, etc... in your negative list. Not only does blocking these terms make your ads more targeted by blocking out irrelevant searches, but by targeting your ads you'll also help to increase your CTR in a good way which rewards you with lower CPCs.