On iphone sync, can I skip transfer of purchases from iphone to computer?
Yes. You can disable app syncing.

To do this:


With your device connected to iTunes select the "Apps" tab.
Under the app listing, deselect "Automatically sync new apps".




New apps will no longer sync with iTunes. They will still backup to iTunes or iCloud, however, so you have no need to fear losing them should your device information be lost.

Note: all instructions provided work for iTunes 11.