Insofar as stamps are concerned, condition is critical, and I do not believe a phone camera will suffice to show what you really have in terms of condition (centering, the condition of the verso, etc). For that, a flat-bed is best.
A consideration to think on:
Scanning (or photographing stamps) can be a time-consuming operation.
You might want to consider getting an inexpensive flatbed scanner or multifunction device for this sort of operation and then coupling it with the freeware PMGS Stampfix offered by one of the forum participants. It would let you place a bunch of stamps onto the flatbed, scan once, then break the individual images out into separate savable files (this is a crude and hasty summary of functionality).... This could save you an enormous amount of time.
Search the forum for stampfix or just go to http://goscf.com/t/47957&whichpage=11
for a link to the latest version
On the other hand, if you are talking about imaging just a sampling (say a couple dozen) items to send to dealers, then this solution might be overkill.
Hope this helps.