Hi
Here are some suggestions/ideas
If you get old albums,with a fair number of stamps in them, rather than removing stamps, either sell them whole, or just take out the pages with the stamps still on them and sell them by the page. I've seen this done in an antiques shop. That way a buyer looking for a specific country can see all the stamps, and you are saved from removing them from the paper.
Loose stamps - some people collect the GB definitives showing the Queen's head - these are known as Machins, and some collectors will buy these in their 100's to look for different varieties. You could separate these out and sell these in bulk.
I know some charity shops have volunteer experts for pricing collectables such as china and books. Perhaps you could contact your nearest stamp club and see if a knowledgable person would volunteer to help you. (Also good advertising to local collectors!)
Have a look on
ebay to get an idea of "kiloware" prices. Perhaps you might like to sell kiloware online. there's lots of charity kiloware on there, and depending on your customer base in the shop, it might sell quicker online.
Collectors vary so much, I think the best answer is to have a variety of packets available, so that there is something for everyone. Countries is probably a better category than continents, as more collectors will look for specific countries.
Finally, as you sell them in the shop, try to gain feedback from the customers, as I suspect you will develop a regular clientele amongst local collectors.