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The best way I've found to make player stocked shops is with Forestry's trade stations. The only Forestry for 1.5.1 is beta right now though, and you would have to edit the config files so that the forestry ores don't spawn in your world if you want to make it so players can't craft the other machines.
If you find something better let me know, I'm always looking for a better system myself. =)
Edit: Apparently Sengir made an official release for Forestry last night while I was sleeping.