General advice requested.

[Sean M4]

I'm new to numismatics and thought the best way to get started was to buy a lot of uncleaned coins. However, only 50% of the coins (the tiniest ones) were actually uncleaned.The larger ones had been pre-cleaned and were slugs. The seller was clearly just wanting to rid himself of his rubbish. Is this common?

[Ron C2]

It's common if you are buying from someone who is not a volume seller of uncleaned coins. 

Uncleaned coin lots have started to dry up around the world as the number of metal detectorists in Europe peaked a decade back. 

Most of the "better" uncleaned sellers bought thousands of coins 20 or more years ago and are still selling from those lots.  Most will be 4th century+. 

In the early days (think circa 2000), many coins in an uncleaned lot were in really nice shape and just needed a little low-intrusion cleaning.  Sometimes just a soak in olive oil and a light toothbrushing.  Those days are largely gone and the current people having success with uncleaned lots are buying top grades and cleaning more mechanically under magnification with scalpels.

Even the better bulk sellers sold their best grade coins long ago.  If you buy a lot now, you'd get many heavily encrusted mystery coins, the odd very common fel temp or provincial, and a bunch of obvious slugs. 

My advice - skip the uncleaned lots these days, spend a little more on properly cleaned and identified coins - you'll actually save money over the long run.