Figure a high percentage of stamps are from India , Israel, Russia and Eastern Europe .
India and Russia I could deal with; it's Hungary, Romania, and Czechsolovakia that are the real killers IMO.
Floortrader, I never bought a lot like that, but I have seen some smaller lots that fit your description to a tee. I always steer clear of that stuff, because regardless of what people think, it has been picked through, especially where these collections are aggregated into boxes. The likelihood of anything sneaking in, is almost 0. Collectors fall into 2 basic categories, bargain stamps, and premium (say anything over $10). If a dealer or someone starts to spot a section within a collection of higher-value items it becomes noticed really quickly. A cert pops up. Say you flip to the columbian page of US, and you see 1c,2c used, and not much else. Then you know, where the collection falls. Same like you mentioned with the UN/Israel stuff. But if you see mint higher denominations, then you are like, hmmm. whats up elsewhere. You can do these observations really quick if you know where higher priced material falls on pages, particularly 19th century. It is so rare, that a cigar box or something got overlooked w/ higher priced stuff, but that is needle in haystack level. Generally by the time it made it to the auction house, it was combed and recombed for any areas of 'value'. I am not saying a find isn't possible, but generally speaking you aren't going to get lucky, but the auction house is betting on you thinking a lot that size, will have gems hidden.
In all candor, unless it is a pedigreed collection, you should assume that any collection or carton lot from any source has been picked over at least once if not numerous times before you see it. If you cannot view the lot in person, purchase or bid based solely upon what you see in any pictures, not based on what is implied MIGHT be included... which is where I think some people get burned. They buy based upon what ISN'T shown. Assume there will be unseen faults/flaws and choose accordingly. Also, personally I would never sink any major amount of money into such a lot without viewing or working with an agent. A few hundred bucks on a whim just for fun, fine, but blind buying this type of material for major amounts of money isn't prudent IMO.
The big sellers also pick the material up and save many hundreds of dollars on shipping.
Not just big sellers. Last month one of the unsold lots at Rasdale piqued my curiosity from a dollars-per-pound perspective, understanding it would likely be doggy poo, but the cost was nominal... except for the shipping cost, which would have been about 1.5x the cost of the lot + buyer premium ($280 to ship a $200 lot, yikes!). So instead I drove up to Chicago to their office and combined the trip with some other business stops. Out-of-pocket cost for gas was about $25 round trip, and the mileage is deductible.
Makes taking blind flyers on bulk lots much more viable. The east coast auction houses, not so much.