I have found some series 5 mini figure bags still for sale at a store near me, should I buy them? Series 6 is also there.

No, unless you want any of the minifigs from those series for your own collection. 

If you want them for your own collection then sure. If you want to invest then you will have to be pretty patient and sit on them for a long while. I can see the Gladiator and Evil Dwarf being worth some decent bucks down the road. Not sure about the others in Series 5 but you never really know. Someone may want the Graduate to commemorate their own graduation. And Series 6 is full of great ones like the Roman Soldier, Minotaur, Classic Alien, Clockwork Robot, among others.

I found a bunch of Series 6 at my Target the other day, bought all 10.  Already had most of them, but was able to get a Minotaur, Skater Girl, Space Girl, and Highlander that I didn't have yet.  They won't appreciate much, but the others I can use for trade bait.