Print parts. Sandpaper carefully the outer rotor and outer rotor housing such that they will rotate buttery. Sandpaper the contact faces of bearing housing, rotor housing and ports housing. It is important that all pieces to make smooth contact. Cut gaskets from some soft rubber and mylar. Obviusly everything must fit perfectly to stop leakage.

Additional items: 1 x 606 bearing, 3 x brass tubes (6mm OD, 5 ID) and 5 x M3 nuts and bolts. Two of the brass tubes are used as hose adaptors and one for bearing axis. Also you will need one M5 bold which accomodates the inner rotor and is used to transmit the torque to the rotor.

Air, water and any other low density fluids will not be pumped because ABS or PLA are too porous and tolerances cannot be made very small. However viscous fluids like chocolate are well pumped. Such kind of Gerotor type pump can achieve high pressure.

Thinner the inner and outer rotors less volume will be pumped so it is easy to design very low volume pumps as needed in precision fluid delivery.

Attached is 3D pdf file so you can better inspect the pump.

Also this pump can be made out of plexiglass by laser cuting the parts. Dxf files will be posted soon.

UPDATE 1
Pay attention to the ports. They should be somehow made "waterproof". Even solids can penetrate in the porous ABS. I am using some paint now.

UPDATE 2
Hey guys! A tip to smooth the rotors out! I have pumped a mixture of water and Ceramite (a kind of ceramic powder used to make moulds). Guess what? Every piece of the pump was sanded smooth. Wash, dry and oil the 606 bearing ;)

UPDATE 3
In search of a "silver bullet" gasket: low shore silicone+adhesive PTFE foil to reduce friction. Any other ideas? :D

UPDATE 4
I have changed the gaskets.dxf file. The scale ratio was incorrect. Now fixed. Units: mm

UPDATE 5
Solved with pump sealing. I have painted the ports gallery, ports housing face and bearing housing face with some nail polish (acetone based). So there is no more porous material. Gaskets were cut from PTFE coated textile self adhesive material. PTFE is an excellent dry lubricant and has good wear resistance. The only spot were air could enter in pump body is between M5 bolt axis and the brass tube holding the M5 bolt. This was solved with some soft rubber o-ring.