For what purpose do you want to buy the hardware? Home lab, production, ...?
There are a lot of shops selling refurbished server hardware on Ebay for example, but there are some things to consider. If you get a used/refurbished HP Proliant which does not have a support contract with HP you are going to have trouble to download latest BIOS and/or Proliant Service Packs (SPP).
I would prefer a vendor having customized installation ISOs for VMWare, which I think only DELL, HP and IBM have (maybe even Fujitsu, but not fully sure about this).
Check the specs of your desired server model reagrding storage controller and built in NICs if they are supported by either native VMWare or by the server vendor.
In general I would get more RAM, 8GB are not enough as ESXi takes about 4GB of them leaving 4GB available for VMs.