I used to buy tickets from scalpers pretty regularly. Now, I would only do it as an absolute last resort. I either a) use other means ( and there are many ) or b) watch the game on TV because I don't give a shit anymore.
I am going to pass on a few words of wisdom: scalpers are not your friends. Sure, they may call you 'buddy' or 'kiddo', but don't think for one minute they're not gonna try and sell you seats inside a stainless steel freezer unit for $350.
If you're seriously gonna buy the tickets from some red-faced guy in windpants with a gold chain as thick as my arm, consider the following:
1. Make sure the date is correct on the tickets
2. Make sure the seats are together, as in, "the same row" or "not separated by five sections"
3. Watch the thumbs! When Shady McScrubberson is showing you the sweet, sweet Yankees tickets, he may be hiding the "OBSTRUCTED VIEW" part of the ticket with his big fat thumb!
4. You CAN haggle a little. Don't be afraid to offer less than what the tickets are being offered for.
5. Ask to see the seating chart or have one with you. THEY ALL CARRY SEATING CHARTS!!! DO NOT JUST ACCEPT HIS WORD THAT THEY'RE 'GOOD SEATS'.
6. "Behind the dugout" is much, much vaster area than you think
7. If the price seems too good to be true...
8. "I'm gonna be honest with you, I'm selling these tickets real cheap because I've been here since 10, I'm tired, I just wanna go home..." LIES!!!!
9. Never,ever buy tickets for the section known as "BENCH".
All of these tips come from experience, and yes, I have been scammed. Teach your children well.