300K Budget build advice

In around August or September this year I'll be assembling a new setup. Need some advice, mainly on the current market value of my current system and places to get components. Main use is for gaming.

Current PC specs:

Intel Core i5-9400, Nvidia GeForce GTX 1070 8GB (ROG Strix), 16GB DDR4 RAM @ 2133MT/s, 256gb SATA SSD. Hoping that I can get at least 100 or 110k for this.. also suggestions on how/where I can sell this would be nice.

Depending on how much I can sell the PC for, the total budget for the new system will be around 250k-300k. Pretty straight forward, I need a PC and a single 120hz 1080p monitor, size doesn't matter. My main objective is price for performance, therefore, I don't mind used components but would prefer if there was a small warranty given for the components (3-6 months ideally). Open to Intel, AMD, and Nvidia components. All comments are much appreciated!