last year i bought HP Z800 12 cores Xeon server
Well, strictly speaking, the z800 is a workstation, not a server (HP's servers were called 'ProLiant' until the whole part of the business was spun out to HPE). It supports ECC but lacks any of the features like remote management, hot-swap capable drive bays and fans, or redundant PSUs which are normally found in servers.
Back then the z800 was different from other dual processor workstations (i.e. Dell Precision T7500) in that it used two 5520 chipset PCHs to make all PCIe slots available even in single CPU configurations (other workstations couldn't use the slots assigned to CPU 2 if only a single processor was installed). Unfortunately HP didn't do this for its successors.
If you need a server then I'd look at the ProLiant series. Which often are also cheaper than a workstation

i was afraid i will miss out, but it seems there are still plenty Z800 in ebay to choose from.
Indeed, but that is because it's been obsolete for several years and it's Nehalem and Westmere XEONs are affected by several major security flaws for which no microcode update exists.
In addition, the z800 has a weak spot which is the proprietary PSU, which over the lifetime has proven to be unreliable and prone to fail. The successor z820 (which uses the same physical shape for the PSU) got a redesigned power supply which was a lot more reliable (and which was also used in the z840), but this one doesn't work in the z800.
If I had to buy something today then I'd rather opt for the HP z620 or z820 which use Sandy Bridge and Ivy Bridge XEON E5 processors (all which are notably faster than Nehalem and Westmere) and which not only are mmuch more reliable but also offer goodies such as USB3, UEFI and even NVMe boot support (retrofitted by HP through a BIOS update).
My recommendation would be the z620 since it's less bulky, also all its PCIe slots are connected to the primary CPU so they all are usable when only one processor is installed. I'm using one as my gaming PC (1x XEON E5-2667v2, 64GB RAM, Micron 9200 PRO 3.8TB NVMe SSD, Geforce RTX 2060 Super)

Thanks to early E5 servers and workstations being retired, prices for systems and especially processors have come down dramatically. it's probably the current sweet spot in terms of price/performance right now.