A collection of miscellaneous items sold together. A collection of odds and ends.
A large amount of items sold at low cost. Usually sold in an attempt to reduce stock of a particular item. In modern day America, we're probably more likely to refer to it as a bulk buy. Some one who buys job lots from a manufacturer to resell is called a jobber.
In the trading of commodity futures, and job lot refers to a contract dealing with a trading unit smaller than usual. Western Barley futures, for example, have contracts that are normally traded in 100 tonne units (a round lot or board lot), but can be bought in 20-tonne units (a job lot). This creates a market in which smaller investors can participate along with the bigger traders. If you are trading stocks, it's referred to as a odd lot (usually a purchase of less than 100 shares), not a job lot.
I've not been able to find much on the origins of this phrase. Job, in the relevant sense, derives from the Old French gobe (morphed into gob before becoming the modern day job), meaning a lump. Www.etymonline.com also gives 'cartload' as a definition of gob. Job lot would then be a lumping of stuffs together -- or a cartload of stuff.
The 'bulk buy' sense might come from the more common 'piece of work' definition of job. A factory or workshop would have a one job (i.e. rubber ducks) one week, and another the next (i.e. little Ernie dolls). A job lot would then be a lot that came from the same job.
And finally, my favorite etymology, that I thought up on my own and is quite probably not true; 'job lot', in the sense of a large pile of random objects, may be a corruption of 'Job's lot', and of course, no one would want Job's lot (I'm referring to the biblical Job, who was one of the more tormented men in history).
In my experience, the "miscellaneous items" definition is used more in the UK, and the "bulk buy" definition is used more in America. This observation is generalized from a very small sample, so take it with a grain of salt.