Sometimes the mob drops str+int or agi+int gears. I put them on the AH for a random price. Surprisingly they sell. My question is which class/specs need these stat combinations? I read the stat priority guides but couldn't find any.
I don't believe there are any specs who would prefer those. There are several reasons that I can think of that people might buy them though.
Firstly if you put it on AH at lower than vendor price then somebody might have bought it just to sell to the vendor.
Secondly an enchanter may have bought it to disenchant either for leveling or because the materials gained by the DE are more valuable than the original item.
Lastly it is possible (though unlikely) that even with one useless stat the item is the best for that level on the AH at that time. This is very unlikely but it is at least plausible.
There may also be other reasons but this at least shows that it isn't pure insanity necessarily. :)
Edit to add:
Ravekner has pointed out in comments that it might be purely for the design of the item - they want it for transmogrification.
It's basically leftover from a time when the specs were different. For example when hunters had mana pools (and mana scaled with int) they would go for agi gear but also needed int/spirit to support their mana.
People buy this stuff because either A) its better than anything else on the auction house at the time for that level, or B) because it has transmogrification value.
You will stop finding gear like this at higher levels (after 60 I believe).