Congratulations?
Depends on what it is. Right now mid-range to high end GPUs are in high demand, so certainly selling that off might be worth it. Same is true of good power supplies.
Parting it out is generally the better idea, not very many people are going to want the exact system you have built.
CPU/Motherboard can often be sold as a pair. With or without the memory.
Shipping an assembled computer is also outrageous, the volume and weight can quickly eat into the sale, and people buying used systems are generally not wanting to pay high shipping. Also a high chance of damaging an assembled system unless you take precautions. Removing CPU cooler and GPU are the minimum steps. Or getting the self inflating packing foam to put inside the chassis.
So, sometimes, if you can find a local buyer, that can be best.
Ebay of course, though most protections go towards the buyer.
Forums. Here on Tom's there is a classifieds section. Reddit/com/r/hardware swap, etc.
forums.tomshardware.com