for a realistic appraisal of the stamp market:
there are stamp catalogues and websites pricing stamps for sale.
This raises the obvious issue - a stamp dealer needs to earn a living, so there is a huge difference between a selling price [to a stamp collector] and a buying price [from collectors].
This is subjected to the laws of supply and demand, just like everything else.
so if you want to sell a collection, you will get a good price only if you have stamps that are in high demand from other collectors.
Unfortunately, the vast majority of stamps that have been issued since the first one appeared in 1840 are not worth much. only a small percentage- say about 5%- are valuable.
a definition of valuable stamp: common stamps are the ones that are value at $1 and less per stamp
a stamp valued at $1-$20 are only slightly better.
You have to remember that 100 years ago, when inflation was low, before the age of the phone, internet and discounted postage, letters were the main means of communications- people wrote to each other fixing a social meeting, and official bills arrived in an envelope bearing a postage stamp.
so, stamps were issued in large numbers [at least several millions], for genuine postal use. today those stamps are worthless. the British Penny red of 1841 can be bought for about 1 Shekel in my local stamp market- because so many tens of millions of them were printed. The local postal rate for letters was 1 penny.
Higher denomination stamps [1 schilling = 12 pence, 20 schilling equals 1 pound] during the 1840-1903 are dearer - because not so many stamps were used - values above 1 schilling were used for parcels or telegrams.
Even in 1914 there were rare stamps, but you normally had to pay good money to own them; it is rare for ordinary stamp collectors to come across a valuable stamp just by chance.
So, under normal circumstances, an average stamp collection would not have valuable stamps.
Unless the collection owners paid good money for valuable stamps in the past, a collection based on stamps from incoming mail or cheap stamp packets would be worth very little indeed.
My late grandfather's collection ended up with me- his collection was extensive, but it consisted of stamps taken from commercial correspondence, or buying stamps in bulk , so his collection was old but not valuable.
I attend a stamp market in London every week, so I must have seen more stamp collections than most of the people who write in this forum [no offence meant to anyone]. I am not a dealer but a collector.
The dealers attending our weekly market are part timers- i,e, they work from home, do not have online sites, shop premises etc. so they do not have very large operating costs -like stamp shops, catalogue publishers or website publishers [investing their time to scan their shop online].
therefore, the stamp dealers attending the weekly markets can afford to sell stamps at cheaper prices- and still earn enough money as the 'large players'.
I can buy the stamps values under $20 each for between $0.05-$2.00. so they must have paid $0.01-$0.20 for the stamp- an even less.
If you buy in bulk you get a hefty discount!
to sum this point up:
stamp catalogues and large stamp dealers [the ones that have the resources to publish a catalogue on line] tend to assign unrealistically high catalouge prices to very common stamps.
So what can you do?
Look at the stamp collection: start with your own country's collection- i.e. Israel.
In Israel, stamps from 1953-1985 are mostly common, and there is more supply than demand, so the value of the collection depends on the price you will get if you sell. expect the price to be low- up to 20% of the catalogue value of you have stamps with tabs which are unmounted mint.
Stamps from 1986 onwards have a 'floor' value in the sense that they can be used for postage.
Stamps 1948-53 are valuable providing that they have the tabs attached.
You can always get an impression of stamp values by searching ebay for stamps from Israel, or a site like delcampe.net.
You can then extend your search to other countries.
once you get an idea of what sort of stamps you have, you can estimate the collection.
if you find that most of your stamps have very little value per stamp, and you want to sell them,
, you might be lucky if you get 0.01-.0.05 per stamp.
My own advice is that it might be better to sell directly to another collector- if you can find one.