I often ask for provenance for the coins that I have purchased. Only rarely do I get anything and then it is something like from "a reputable German dealer" or "an old Swiss collection", which doesn't do much for me.
I've had the most success with ACSearch finding previous auction listing for coins that I own - here you can go back a few years, but usually the provenance is limited to another auction house. Researching a coin I've also had some luck finding coins that are highlighted in articles or old catalogs. Here's a nice example: AE Coin fromEkkarra, Achaea Phthiotis
There's not much magic - as much as image search may eventually prove more useful for provenance hunting, I personally have had limited success with Ex-Numis and ACSearch image search. I think there are a few hurdles still to overcome in content (building better databases) and search technology (better algorithms for recognizing die matches and exact matches).
My favorite example of "surprise provenance" is this one. The CNG source was listed with the coin, but surprisingly no mention of the Shore provenance:
Phraatakes 2BC-4AD AR Tetradrachm 11.38g
Obv: Bare-headed bust left without royal wart, with medium pointed beard, wearing diadem, earring visible
Rev: King seated right on throne, Tyche standing before him, giving him diadem and holding cornucopia, standard legend, below Tyche's arm IT, above Tyche's arm, IT, ΓOPM(?)
Ref: Shore 312 (this coin) CNG Auction Catalog
More notes on this coin are here: "Parthian King Phraates IV and Son"
What can I learn from the coin flip than came with this coin? Is something like this worth keeping?
This coin is a relatively scarce quinarius of Augustus, the first Roman emperor.
Augustus, 27 BC-AD 14, Quinarius, Emerita, P. Carisius, legatus pro praetore, 1.7g, 16mm, die axis 6h
Obv: AVGVST, bare head right.
Rev: P CARISI LEG, victory standing right, placing wreath on trophy; dagger and curved sword at base
Ref: RIC 1a
The card that came with this quanarius, doesn't tell me much, but it may have a few clues to the previous owner or dealer. The first thing I notice is the £100 - from which it seems this came from a dealer in the UK.
The second is S. 402 which is a reference number from David Sear, but doesn't match my 1988 copy of Sear's Roman Coins and their values, so it could be older than 1988. The other reference is Seaby 386 which is consistent with my copy of Seaby, but this reference is usually described as RSC (Roman Silver coins in recent auctions) and the numbers in my 2004 reprint are the same as the 1978 3rd edition.
Short of seeing another card that looks similar - that's about all I can get from this card which is still worth keeping filed away as useful provenance for the coin along with the person I purchased it from.