Cleaning the coins with a soft cloth no matter how soft will leave tiny scratches called hair line scratches by collectors. I wish I knew what you meant by crud? Proof coins have a special surface and usually anything done to it will make it change maybe not now but in to months to come. On the other you can try acetone the kind from a harware store 100%. Dip it into that for it will desolve glues and goos as well as oils and grease. put coin in distilled water and pat dry with solt cloth no rubbing. try that and see do one one and check it out before dong the rest. I do not recogmend cleaning proofs but you can try one of the less valuable ones. The one problem of doing business with ebay is the many crooks on it. In the end the average Joe does not make out. I actually get better deals from my local dealer. Hope this helps.