They are usually used stamps removed from envelopes by the members of
churches, scout troops etc to raise money. You can expect to get a lot of
common stamps in them and these lots certainly vary in quality - some folks
enjoy getting hundreds of copies of some common stamp so they can find
postmarks, printing flaws and so on.

I'd happily give a mission mix a crack - I've bought several over the years
and have bought more good ones than I have bought duds. Let's face it - if
you pay $10 for a bag and get 200 decent stamps and 1000 duds, you've still
got yourself 200 decent stamps for $10 which can only be a bargain. If
you're thinking of getting them from ebay, check the seller's feedback
first.

--
Cheers - John Mycroft
coryton_at_cobbsmill_dot_com