Other than asking you for money in order to providing a service, what are some of the dubious tricks agents and publishers get up to (obviously I’m talking about vanity publishers here)? So long as you don’t sent money to an agent/publisher, or sign away copyright, I don’t see how you could get too badly stung.
If you sign away the first-publishing rights, you're unlikely to get the piece published anywhere else.
If you don't get a firm date of publication in the contract. If there is not a solid reversion of rights/out of print clause, a way to end the contract with an agent after a period of time, limiting the right of first review for the next work written by the author, non-competition clauses, granting rights that the publisher has no record of using, no intention of using, any staff or experience to exploit/sell on behalf or the author, claiming to have distribution and placement in bookstores and actually doesn't, if a contract lists royalties based on net, how is net profit defined (what expenses count against it). These may not be 'scams' but they are just a few things that an author should be aware of as they deprive (cheat may be too harsh of a word since an author should read and understand the contract even if represented by an agent) of fair compensation for his work.
legit agents don't use 'tricks'... they simply work at getting a publisher to take on your ms... for which they then get paid 15% off the top of all you make on that book... and it's usually well worth it to the writer, since the agent can get a much better deal... see the facts about agents here: http://www.invirtuo.cc/prededitors/pubagent.htm for publishers: http://www.invirtuo.cc/prededitors/pubwarn.htm always check that site first, before contacting any agent/publisher... but don't stop there, if it's not listed, because dave can only list those he gets feedback on... so also check the company's website carefully and google their name for any negative feedback...