But it depends on the coin and the price, no? Suppose he was playing with a bunch of Morgan dollars with his greasy fingers and was happy to sell them for $5 each? Then the question is a valid one, no?
For $5 each, I'd gladly accept a few greasy Morgans. If nothing else you'd make a killing on bullion value alone. If you come across a yard sale where this is happening, let me know....I'll travel Guy~
Think of it as a Rescue mission. Rescue the device and in this example you can eat it too! I once tried to point out to a couple at their yard sale that the "lot in a box" price included a WWI Mauser bayonet worth more than the $10 they wanted for the whole box of "Stuff". She was adamant~ "I don't CARE! Just give me $10 but ya gotta take all that junk outta here!!!!" So I did. Got $70 for the bayonet elsewhere. ps~Now I still have a box full of junk from the purchase to dump....minus the two grungy standing liberty quarters I soaked apart to discover they are unreadable and are therfore bullion. Still not a bad deal.
I'm going to have to start asking people if they have coins at yard sales!! Best bargain I ever got at a yard sale was a couple hundred dollars worth of art supplies for $5. I kid you not. I did and still do use some of those supplies! It didn't look like much on the top but as I got home I dug down into the box, and the expensive stuff started to surface.
I have yet to see coins at yard sales, I have seen old coverless comics that the seller thinks is worth a lot, but I have never wanted to set them straight, let them dream I guess.
I got a fishing tackle box just loaded with matchbox cars for one dollar. There are 4 airplanes in the mix too. One diecast airplane is identical to an ebay listing for a hundred dollars. Lots of the matchbox cars are new. The tackle box alone is worth the dollar.
The type of things that are "safe" to purchase at yard sales are junk silver, bullion coins and mint/proof sets in the original packaging. I don't think you will find a TPG slab, and raw coins with grease on them shouldn't be bought at all.
I'm going to assume a collector accidentaly put his french fry fingers on a unc silver coin and wants to know how to fix it. Either that or he got a silver coin in change at Micky D's and wants to perform preventive mantenence.
I am going to assume that the hypothetical coin has been somewhat circulated in the past 45 to 150 years or more since it was minted and has seen quite a few hands and pockets. Therefore, a soaking in a warm soapy water (no rubbing whatsoever), cold water rinse and light patting with a towel is surely not going to hurt the coin! This should remove or neutralize most if not all of the oil from the french fries. Once done and to be on the safe side, a light bath in Acetone should remove any possible leftover residue of the oil. Frank