Well, here's how to go about it. For purposes of valuing the estate for tax and probate court purposes, someone (the executor or his/her agent) will need to inventory all of the assets of the estate and assign some sort of valuation.
This is hard work and there is no shortcut. Whoever does it needs to start a spreadsheet and start listing all of the knives (unless they are obviously junk, in which case they would still need to be listed as "700 assorted pocket knives, approximately $1,400 value". So make, model, condition, all need to be taken into account. Once he has compiled the list, then he needs to get an appraisal or estimate of value for each item or group of like items.
I would go with the expected value that they could sell for, since if you have to pay creditors and divvy up the estate among beneficiaries, you'll either have to sell them or assign a market value in case one of the beneficiaries wants to keep some of the knives. You would have to allocate the cash value against their "share" of the estate.
I suggest having the executor of the estate pay for an official appraisal. if the OP doesn't want to go through the hassle of photographing, listing, and selling each knife individually on ebay then the other options are an estate sale, or finding a broker who will buy the entire collection (usually for rather less than they could be sold individually).
If it were me, I would at least get a basic list of # of knives, manufacturer, and country of origin (like 10 Kershaw-USA, 20 Boker-China, etc) just to get a basic idea of how much effort they should expend.