I have a question. Is it illegal to buy 100-100000151MPK (Ryzen 3 4300GE) CPU for DIY home PC?
No, but the Ryzen 4300GE is a OEM processor and MPK , I believe, means it comes in a MultiPack package.
So you are purchasing a OEM/Tray processor that has no AMD Warranty except through the Retailer/Vendor and doesn't come bundled with any CPU Cooler or in a AMD Retail Box.
Some Retailer/Vendors do sell OEM/Tray processors to the public like this foreign Retailer: https://greatecno.com/gb/processor/179087-amd-ryzen-3-4300ge-mpk-multipack-processor.html
The above Retailer gives a 2 year Warranty whereas AMD gives a 3 year Warranty on Official AMD Retail Box processors.
Also you need to check your Motherboard's CPU SUPPORT LIST and see if that APU is listed as being supported and compatible.