Values vary depending on who is selling.

Hi, I have inherited a large collection of coins of various dates but mainly from the UK.

Florins, Sovereigns, shillings and many more which im looking through.

I hear people ask what's the value and alot say that its how much it's worth to the person but as im selling these for my friends wife(he passed away in October) and I'm not a collector what's my rule of thumb with valuing as I don't want undervalue for her or overvalue and not sell. For example there is a Morgan dollar dated 1888 which has an 's' mark on just above the D and O? Numisma says about £40~£50. Any advice is greatly accepted.

A very important step in valuing coins is determining the grade or the general condition of the coin as it can massively affect the value of a coin. Here is a wikipedia article on how to do it properly. https://en.wikipedia.org/wiki/Coin_grading

Researching the likely value of a coin takes some time.  

For coins which are somewhat valuable  (say, 100 and up), you can use coinarchives.com , which is where the search on your Northumberland shilling came from.

Below that, you can do an Advanced Search/Sold Items on eBay.  But remember the asking prices are useless.  You must only look at coins that have actually sold (which are always far fewer).

You compare your coin to coins with similar condition that have sold recently.  It's not that hard to get close, unless the coins are in top condition with very few blemishes.

There are also companies that will sell  your collection for you.  If it's indeed a large collection, this might be the least stressful avenue.