Pre-season NFL betting can be very tricky.  The real trick is to know exactly when the coach will pull his starters. And the truth is that sometimes the coach himself doesn't even know when he will pull them. If the starters play well and get accomplished what the coach wants to get accomplished, he will happily remove them and shelf them and let the backups get some PT.  The point spreads are calibrated from multiple factors:

1 - The strength of the starters
2 - Location of the game i.e. home vs. away
3 - The strength of the backups, including rookies
4 - When the starters for each side are "supposed" to be pulled

The typical difference in a spread can be several points when comparing regular season with pre-season.  Ex. Let's say Dallas travels to Buffalo in pre-season. The spread will more than likely have Buffalo slightly favored because of the backups and the fact they are at home. Let's say Buffalo -2.5 vs Dallas at home. But if they met in the regular season, you can count on Dallas actually being favored on the road because of the more talented starter base. Dallas would be -3.5 or so.

Some handicappers say that there is a much better chance to make money in the pre-season. Whereas others say that it is a crap shoot and a waste of time and money. I think the truth is somewhere in the middle.  If you are well informed, and bet pre-season football, you may win, but play conservatively and tread lightly.  Pay attention to pre-season games as you may see glimpses of the stars that will emerge during the regular season.