There is no single right answer. Too many variables. Too many different buyer goals/objectives. Is the buyer in question:
1. A collector buying solely for expanding their personal collection.
2. A collector looking for certain items to keep and then flip or trade the remainder.
3. A collector just "looking for some fun" to pass the hours; sheer enjoyment.
4. A part-time or "vest pocket" dealer looking to break down into individual lots for resale.
5. A full time dealer looking strictly at wholesale cost vs. retail opportunity.
... or any combination of the above.
At various times, and more importantly depending on the country/specialty, I could be #1 through #4.
An important (in my opinion) concern for anyone is to assess conservatively, especially if you are purchasing from images rather than examining in person. In-person buying is a completely different animal. For the pursposes of this discussion, I'm assuming the purchase is being made online, since that is what the OP mentioned.
Assessment methods/approaches vary. You never know what flaws might be lurking, or even if the stamps are what you think they are. For example, depending on the makeup of a lot, I might tally up the clearly-identifiable catalog value, then divide by 2 or 3, and then try to buy at 10% or less of that figure.
Conversely, if it is a highly-specialized collection I may do more thorough analysis and calculation, factoring in keepers versus what I might garner from
ebay or the APS store on the remainder once broken down (or flipped en masse).
If it is a messy stockbook lot, it may be more of a ballpark or "gut feel" for how to assess.
Also, when we say "collection" are we really talking about a collection (presumably organized to a certain extent) or an "accumulation" (a hodge podge... i.e., a mess)?
For example, the two lots below are on their way to me right now from overseas. From a dealer/wholesaler perspective I obviously overpaid, and realized that going in, but I also factored in the "fun factor" for me personally, as I absolutely love lots like these. I collect SOTN cancels on classic worldwide stamps, so lots like these are fertile hunting ground for that kind of material. They also provide a great source of singles/sets to put on
ebay and to put together packets for my local stamp club.
Another reason I like lots like these is that once I remove the items I want to keep, if I take the time to redisplay the remainder in a more organized/cohesive/presentable fashion, they can frequently garner far more upon resale, as right now who knows what the real quantities are or what is hidden? It's a lot of guesswork.
Admittedly, part of the allure for me is the gamble... better than buying lottery tickets or playing craps, I suppose.
However, they are also some of the more difficult sorts of lots to assess from a value perspective.
If you were considering buying, how would you go about assessing them based solely upon the images shown?
Lot 1: Sweden






Lot 2: France




