Any advice for a newbie like me?

Go slow and get what you really want. I made the mistake of getting stuff just to get stuff. That is not the way to go. Luckily I've been able to sell some of the stuff for only a small loss, but it's a pain in the ass. Now I am much more focused. Yes, I still get some cheaper stuff for fun, but mostly now, I am concentrating on what I really want. 

When buying used, buying from a trusted seller is the single most important consideration in my opinion. 

buy the seller before you buy the watch 

The advice can vary depending on the watch you are going after. Stick with reputable dealers and do your research will always apply. I never pay over MSRP on anything, and honestly unless it is a few select brands or models, I don’t pay MSRP either. 

Lots of good threads here on good resellers, etc. 

Start here

It depence on the modell and vallue.

Rule No1: Don't trust anyone!

Every offered watch has a reason to get sold. Typical answer of the seller: "I used it rarelly. It's allmost unused. It stayed all the time in the box, so as there is no use I'm about to sell."

Another good one:

"I need space in my collection for new one, so I decide to sell this one!"

I don't want to blame everyone, often it' true, but many get the Idea of selling when it's actually time to send the watch for service.

Depending on the brand, the complications, the age, the possible problems and the last service (probaply never did before) this can cost you easily as much as a good middle tier watch.

That's because many people bought expensive watches with their last money but can't afford the maintenance.

Same when the watch has some issues

The golden rule: if you can't buy two, you can't afford one.

My tip: Ask a professional jewelery if they can make at least a test of the accuracy. Mostly you can recognize there if it has a issue. It might cost you in worst case few hundrets but ut will save your ffro even more worst costs.

Ask the owner for the last service and a proof for the service. At least a recipe. If it's older than 5 years it's only a "nice to have"

Mostly the watches are totally overrated by their owners. Depending on the brand most owners expecting to get 80 to 90% of the retail prices. At least they try to. Often they get allready at retail  up to10% (depending on the brand) which means they expect up to 100% of their payed price back. For a woren used scratched unserviced watch. They love to call it "patina". 

Tip:

Ask yourself if it's worth it for yourself only to safe 10 to 20% or is it better to buy new.

Decide for yourself which watch you really want. Check and follow the market over months or year. Don't worry to miss a watch. There are milions outh there. Over long term following the market you will get a feeling for the real worth.

Make yourself clear, even if it's your grail watch, that you don't need the watch to stay alive. It's just a watch. Sometimes we stay in our bubbles of watch enthusiasts. But the truth is: 95% of humans or even more don't care what's on your wrist.

So make sure you want the watch because YOU want it for yourself, not for the respect of others.

Otherwise you will only looking for overhyped brands or modells which means you will follow the market and have to accept the prices with all risks.

I would recommend you to check on professional pre owned sellers as like Watchfinder. Watchfinder belongs to the Richemont group with a great source of high skilled watch makers and easy access to parts. They check and repair every single watch and gave you a 2 years warranty as I know.

Also chronex can be considered.

You can easily order check on your wrist and send back if you don't like it.

Good luck

Woow thank you so much for this