Just be aware that the seller PC Servers and Parts tends to strip out anything of value to get the price low. When they say barebones, they mean it. On the HP Z440s they sell, this usually means no RAM shroud/fan, drive trays, etc. ...
That might be exaggerating a little. On their "barebones" configs, you will get everything that would be part of the manufacturer's base configuration (less cpu, ram, storage, os). For example, the Z440 and the Memory Cooling Solution--that fan shroud is an HP-"optional" part, only
required_on (else won't POST), and HP-configured with, z440s filling more than 4 DIMM slots.
Another example, the Z4G4 has an HP-optional part, Front Card Guide and Fan, which is only factory configured with systems w/ the 1000w PSU; but required (cf: Z4G4 QuickSpecs) in conjunction with specific single/multiple Graphic Cards. However, when PCSP sold me a
barebones Z4G4 which was advertised as having 1000w PSU, my box did have the guide/fan (which was not mentioned anywhere in the eBay listing).
You'll get what you "should" get; if it turns out not to be what your own add-on components require, that's on you. Your due dilligence should include a careful (even meticulous) read thru the HP QuickSpecs for your model, along with picking out the relevant details in the PCSP eBay listing. That's been my experience.