Here is what you need to know. I have been buying and selling bears tickets for about 10 years.
Cheapest and Riskiest:
Craigslist; most of the time I would say you will be okay, people generally offer them cheaper here because they have no fees. However you're open to extreme fraud, even a hard ticket isn't safe as a season ticket holder can just email reprints to himself and you're sol. Only way to buy on craigslist is to get a signed bill of sale, verify address, and have contact information verified. Anyone legit will have no problem giving you this and it covers you for the most part. Yes you would have to sue if it was fraud but at least you would have a chance.
Safest and Most expensive:
Ticket exchange/Stubhub; you're guaranteed your seats but the sellers pay a hefty price to sell here so expect to pay more. However the experience is stress free.
In the middle:
eBay; again only buy from a verified seller, use paypal AND only use a CC through paypal to cover yourself twice. If shipping is needed insist on a service with tracking, and signature required. Offer to pay more if needed, this is needed to cover you in the case it is fraud. (Buyer and Seller).
Would never use:
Ticket brokers; forget these guys, they are the reason you can't get a ticket on ticketmaster after 10 seconds. However, buying from one on ebay sticks it to them somewhat and you will be safe from fraud 99% of the time. Just wait until the day before and email them with offers for cash.
Avoid at all costs:
Guys under the bridge at the stadium, you will either get a fake or go to jail 50% of the time.
Hope this helps somewhat, back to work for me.