That front end is
here. Whatever you spend on an ex-rental gun, or used parts, remember that you can get a new CM.028 for £78.61 posted from gunfire.pl - for sure, not what you are looking for, but between them and taiwangun.com they have pretty much all the .028 variants for between £80-90 posted, and with them, even though you may not want say, a plastic receiver, or a gearbox, or whatever, you do get all the screws, the bolt cover and rail, the little spring which pushes the rail back against the top cover catch (which nobody seems to sell on its own), wiring which is not only passable but excellent to be reused as the signal wire if/when you want a MOSFET in your new build (because the insulation is very tough), and of course the veritable Kalashnikov of the airsoft world, in that you have to do something really stupid to it to f*ck it up, a CYMA 02 Ver3 gearbox (which can be had for £37.50 but most ppl don't know that and happily pay £51.34).
I'm not suggesting that you do buy one. By all means build your own; fiddling with my guns is almost as much fun as using them to me, just in a different kind of way. I am saying compare the price anyone is offering to what you can have new for the same amount, because as someone else recently stated on here somewhere (Giant Kiwi possibly), the prices which many people imagine their used AEG's and parts are worth are out there with Dr. Sheldon Cooper compared to used sales of anything else generally, market forces pertaining to supply and demand particularly, and just plain rational thought...