The salesperson in a cell phone store is telling me that the phone i'm considering has 8gb of memory, which means i can save 10,000 photos taken with the phone's camera! is the salesperson telling me the truth? why or why not?

The sale person is not telling you the truth. 
This is because, the size of an average picture on phone is about 3 megabytes. One gigabyte is equivalent to 1000 megabytes; so 8GB is equivalent to 8,000MB [Megabytes].
 If you divide 8,000MB by 3MB, which is the size of an average picture, you will have approximately 2,666 pictures. This means, you will only be able to store about 2,666 pictures on that phone compared to the 10,000 pictures the sale person is talking about.