Advice for selling watches

Cash is king...  I've never sold any watch that expensive before, but I've sold cars.  Ive always done it in person and at a bank.  Cash or cashier's check would probably be best.   Crypto currency could also work fine, if you use it.  

Where abouts are you. I know of a chap who has a shop that buys and seal watches. I bought my Tudor Pelagos from him. He works by either visiting him direct at his shop or via secure postage and bank transfer. He’d be able to give a confirmed price pretty much by looking at detailed photos etc

You can probably see the bank transfer into your banking app pretty much immediately.  That's how I sold my last car.

Depends on where you are but there are apps that show you at which point a transfer is irreversible by the sender and there are also apps (like Swish, in Sweden) where the money exchange from their bank account to your is instantaneous. That how I buy and sell everything over here. 
 

there are also websites like Chrono24 and eBay of course, but they typically have terms that strongly favor the buyer, so if there’s a dispute it could be bad for you. It’s hard to beat an in-person exchange in a neutral, safe place. 

There are generally 2 routes you can take:

• Act as a Private Seller - The pros are you set the sale price but the cons are that you have to determine where to list your sale, deal with logistics, shipping insurance, payment methods, etc. Some of those things you might get from an online watch market like Chrono24, eBay, etc., but you'll pay fees to that online platform for convenience (which may or may not be worth it in your opinion).
• Sell to a gray market dealer - This can be a local watch shop in your town or an online shop like Bob's Watches, Hodinkee, etc. The pros are that you often will get a quoted price quickly and are generally assured the monetary transaction occurs in a fairly seamless manner (wire transfer, ACH, cash, etc). The cons of course are that you should expect to get much less than the gray market "asking" prices you see on Chrono24 or other sites (after all they want to profit). Depending on the watch brand and model this can range up to 50% lower than secondary market pricing you see online. 
• I was advised by a watch friend to do my own research on sale prices for similar watches to determine a range one would expect so passing that along. 

In the case of your Rolex DJ41 the Wimbledon dial is very desirable, and assuming it's newish and in very good condition you're probably not going to lose money even if you sell to a gray market dealer (but you might not make money either).