In 2005 I handled the estate sale of two modelers' kit collections. The first step was to inventory each kit and list them all in an Excel® spreadsheet, including manufacturer, subject, scale, condition, aftermarket items in the box, and remarks.
The first collection was sold at silent auction on two Saturdays to the members of the local IPMS chapter. Unfortunately, the deceased had combined up to four kits into single boxes to save space, and severely marked up the boxes. Their sale value was around 1/4 to 1/10 retail because of the condition. Many of the sprue trees were cut up to save space, so a box with, say, 4 1/48 Tamiya Spitfires, which had to be re-sorted and bagged by the buyer (and missing parts identified and replaced), fetched about $10, compared to $28 retail per kit. We negotiated a price for the whole collection and bought it from the widow. We sold some 500 kits in approximately 300 lots. There were only a few items that didn't sell, and they went onto a vendor's table at the chapter's annual contest and swap meet. After deducting the initial cash, we gave the widow 50% of the profit as well. The club netted a few hundred dollars, and the widow got around $2,000 total. We also put her in touch with a local used military book seller, who bought the books. We put in probably 150 manhours of club volunteer time, plus one member donated his living room as a warehouse before the auctions. The widow was a bit disappointed with the total, but at the same time she admitted to busting her late husband's chops repeatedly for cutting up the kits and destroying their value. In the end, I believe she accepted the price as fair. Worse yet, the deceased had habitually marked up the books with underlines and margin notes, in ballpoint pen! Some very desirable books sold cheaply because of that.
After we inventoried the second collection, the widow decided to set up a vendor's table at the same IPMS local chapter contest, and ended up selling the entire collection to one of the regular vendors who works the model swap meet circuit. The key was an accurate inventory with an objective appraisal of condition. The vendor looked at a few samples, saw the descriptions were accurate, and offered a lump sum for the whole collection on the spot. The widow gladly accepted, just to get out from under the hassle of selling each peiece off a table. There was also a model RR collection. One member of the IPMS chapter is also active in a model RR club, and they bought the whole model RR estate.
We advised both widows at the beginning they could probably get the highest prices by selling the items individually on e-bay, but neither they nor the club members wanted to assume responsibility for all the payment transactions, individual packing and shipping, etc. Even though you can charge for shipping on e-bay, you still have to buy boxes and packing material, set up an area for packing, spend time packing and hauling the items to the post office. I'd recommend PayPal or some other way of making the transaction less risky.
As for market value, there are published price guides for older and collectable kits. Currently available kits can be priced by checking online at Roll Models, Squadron, Great Models, etc. Factor in the model condition, whether or not it's sealed, and the condition of the box.
"You can't have everything--where would you put it?"