Added on by John Sturr.

If you are posting on Flickr or 500px or anywhere else for that matter there may come a time when someone will want to purchase/license one of your creative works.

I am by no means an expert here - but the following may help with this inquiry.

Three pieces of information, listed below will be invaluable to the experience.

Advertising Photography Lou Lesko
Best Business Practices for Photographers John Harrington
Foto Quote Cradoc

This is one possible experience.

Email:  Hello this is "Name", from "Agency" and we would like to discuss the possibility to license your photo.  Please contact me at ....

This is really cool, heady, and big stroke to the Ego, contact the Agency as soon as possible as agencies motivations are time sensitive, and this may mean a sale for you - remember they are coming to you - so get back to them.

Eventually there will be a conversation about price - the tricky part.  

1 - Listen to the offer - just listen and don't say a word.  Let the offer come to you.
Do not volunteer anything which may benefit who you are talking too - just keep listening.
(You will probably hear something about "budgets are tight - we don't have a lot for this one - I hope you understand - we always value Photographers...."  See this video - VendorClientVideo

2 - Make sure you understand the requested use and time needed for the image.  Is it Billboard, Magazine or web, etc.

3 - Hide the fact that you are excited to get this call, tell them you will need to run some numbers for the use request and offer to get back to them.  You will be providing them an estimate for work.

Now -- you have some decisions to make - They contacted you because for some reason the image is "speaking" to them to solve their problem.  This may be an all or nothing negotiation - or they may have 10 other options right behind you they will entertain, if they don't get their price.  Keep this in mind.
Pricing is tricy - as some pricing is regional, and sometimes that is hard to determine.

I'm also going to offer this - The pricing dance is different for someone who never purchases photos - take for instance a future bride and groom.  They probably have never purchased photo services before so there would be a learning curve for them to understand the industry.  But - if you are dealing with an agency, or a Creative Director, well -- they should know or have an idea of what is reasonable.  At times you may be able to follow their lead and they may even hold your hand - and the negotiation could be very smooth.

4 - Go read and use the three references above - oh my gosh - I can't emphasise this enough.  The two books will describe in detail, specific situations and examples, and the software will allow you to estimate a price for the use.

Lou Lesko, in addition to his book, also produces BlinkBid software which will create invoicing and estimates.

We, I'm assuming, all want to please those who want services from us - but - as in retail, some come in your shop to look, and some buy - and some leave.  Your price may be just that - too much, and they may just walk away, or you may walk away.

Here's another thought - what happens if I don't close this deal - my work will not be seen and I will loose that opportunity of exposure.  Well, that's something to keep in mind.  These are all difficult decisions - and I would assume become easier to decide when there are 20 jobs in the que.  If you are small - then - you have fewer options to lose this exposure of your work.