from ebay for example, and how much should one pay for a full set

Not from ebay, I wouldn’t. You won’t know it’s service history or even able to have it reliably inspected prior to purchase.
Hit your local dive shops. A lot of dive shops have clubs. Many will have second hand gear that club members put up for consignment sales if they bought a newer bit of gear or they’ve outgrown their normal gear ( donuts will do that to you). Chances are pretty good that the shop will know the history of that gear because they will have serviced it for the seller. That shop will also make sure it’s safe and in good working order before even listing it for sale. It’s in their best interest to do that since they want you happy. Happy divers come back to the shop to buy more gear. Dead divers don’t.
A complete used gear package can go from a grand and up.That’s almost like asking how much to expect to pay for a used car. It depends on the car and it’s condition.
New gear and you can expect the price to start at around 1500 bucks all in. It only goes up from there. I’ve got about 30 grand invested in my two rigs for my recreational gear for example.
Edit: Tony is on my BS list. He likes to BS people and the guy has no idea of what he’s talking about. That would be why he is the only person I have blocked. Period. He’s not dead yet. He should be. He once advocated using a 32 % mix at 250 feet. Uhhh..right.