Every once in a while, I get an offer on a wine collection from an estate sale. I would say sometimes these are great opportunities but sometimes these can be a money pit. Here's one way to go about reducing your risk and increasing the reward...
I would say that if the wine has not been stored properly within 2-3 months then it should be considered passive storage. The discount there to me starts at 40% off wine searcher low. This is how my broker friends do it who sell passive cellars. I do not sell passive cellars, but I do buy them for myself every once in a while, just to drink at home. Rarely, almost never do I buy 1000-bottle passively stored wine collections, but this formula works for collections big or small.
Wine that is passively stored can still cellared for a time period, but generally for every month you store wine incorrectly you take end-life away from the wine, so its aging accelerates. Therefore you start discounting wine accordingly. Here's how I would proceed with this parcel...
2. I would ask the seller to open 10-20 boxes, line up the wines horizontally in a well lit room so you can at least see fill levels. The goal here is to line up the wine so you can see how much variation there is in the fill levels. If you see drastic fill levels differences, then I would run for the hills. The wine was not stored well if the fill levels drastically vary. If you see consistent fill levels on the same exact wine, and the fill levels are top shoulder or neck levels, then that's a good sign. Another reason to do this is so you can use this as a way to determine the strength of the corks. If the wine was laid down on its side in a room with low humidity, the corks will be dry and you will have failures and more ullage. Dry cork is not good as it makes the wine significantly susceptible to oxidation.
3. Ask the Seller to take a picture of where the wines are stored (a photo of the room) so that you can see how the Seller is storing them now. Ask them where the room is located in the house. Do not ask leading questions. Are they in a room with an open window? If so you can expect a delta in the temps and that would reduce the value as aging has been accelerated. If they are in a closet in the middle of the house and its a home that is lived in year around, then thats a good sign.
4. Have him open up 10 different bottles from different boxes and take a picture of the cork. If the wine had heat spikes you will see long thin wine trails along the cylindrical outer layer cork itself. That will tell you if the wine was subject to severe spikes in heat or cold. Severe fluctuations in temps are really bad for wine. If you see long trails or wet cork more than halfway up, then that's not a good sign.
5. If you have a clear line on 2-4 above, then you have the basis for how you are going to make an offer but you still want some relative data points on the wine itself.
6. Have the Seller take out one bottle from each box lay it on top of the box so you can at least see the capsules match the wine, and have him snap a shot of each bottle on top of each box. You're asking him to take 80-100 photos. Thats way less intense than asking them to perform an inventory and the problem with giving them an inventory option is that you may actually be shooting yourself in the foot. It's to your advantage to do it this way.
7. Once you have an indication on the quality of the wine, you can google wines and see relative values and formulate a basis of how you will offer. I would start with 25-30 cents on the dollar. Cash is always king. If you pay cash, then you should get a better deal.
I hope this helps, and I hope there's a case or two of Petrus or DRC in there. Sometimes you get lucky!