>>> "Stamp collectors buy stamps in bulk from us in the hope of getting a couple that might be of value"
Yes. As you're probably aware, it's called 'kiloware'. My father spent many, many hours sorting through modern British kiloware (which consisted of the corner bits of envelopes, with stamps stuck on them). It's a very specialised hobby where what would appear to most people as just one stamp can have hundreds of variations in colour, perforations, watermarks, phosphor bands and much, much more.