What we usually call a "regulator", like the Milwaukee one, is really an assembly of a regulator, needle valve, solenoid valve and often a bubble counter. A regulator from ebay will probably not include those extra parts, so the cost savings that way are diluted by having to buy the extra parts. Still, getting an industrial or lab quality regulator on ebay for $20-$50, and adding a good needle valve gives you the absolute best "regulator" you can ever get, even if it ends up costing a bit more than the regulator assemblies made for aquarium use.