I've been shooting a lot of photographs and they are posted on Flickr.com.
Today I got an email from an art company that said one of their clients wants to buy one of my photos.

I'm just starting out so I don't really know the price I should sell it for and the procedure for completing the transaction.
I would like to keep the rights to use in a portfolio but not necessarily to re-sale.

I would really appreciate any help and advice on this subject.
thanks!