People who've sold coins before or have participated as a seller at coin shows/flea markets/collector markets, any advice for a beginner?

First of all I want to mention I do not want to leave this wodnerful hobby or sell out my collection. Over the years I've accumulated lots of doubles/lots of world coins that don't intersect with the main areas I concentrate on when it comes to collecting. If I can call it anything, it's just a restructuring of my collection, as I'm trying to sell the coins that don't interest of me or that I have plenty of in roder to buy the ones that really spark my interest.

Now, I'll be participating at a monthly collector market that is held monthly in my city where all kind of collectables and antiquities are sold (including coins). I know the question might seem weird, but I want to make sure I don't make mistakes that can be avoided. When it comes to coins, I've been the definition of a pathlogical hoarder, so my selling experience is a flat 0. So, if you have any similar experiences behind you, I'd love to hear some advice, what "strategies" you use when it comes to pricing/negotiating/just general advice.

Thanks!

With your "monthly collector market" you'll contact only your neighbourough in a country with a low purchasing power.
With a professionnal ebay shop, you'll be in touch with the whole world including some rich counbtries as USA or west europ.
Moreover you will get a free year or at least some free months free for a professionnal ebay shop.
The main constraint is to get a good picture of each of your loose coins. Then you must apply attracting price and the most important thing is to offer the lowest shipping costs as possible with all sorts of discounts for large orders.
Some Numista collectors have sold thousands of coins on ebay

First, let me just say that I'm not a very good (nor consistent) seller but I would like to become one (some day). I've sold a few banknotes on a few occasions & I highly recommend collectors selling duplicates & thinning out their collections at least once per year (or more often if they can). I always started out small & then sold a few more items but generally I have never sold consistently for any length of time & I think that might help a lot (try to spread out what you have & try to sell consistently over a few months at least).

My advice would be this: get very organized! Have envelopes & packaging ready before you start selling. As @Frenchlove suggests take good photographs (or lean how to) with decent lighting (& with different angles). You need to: a) list your coin accurately & communicate exactly why it is collectible & b) be as transparent as possible (no surprises for your customers). Whether your grading skills are good or "so-so," it might be wise to also qualify that your description is your opinion & for bidders to judge the coin for themselves. I strive to have NO returns (if at all possible) & discourage new & deadbeat bidders but you can still experience this no matter what you do. I invite communication about the item & try to get back to potential buyers asap. A word of warning, develop a thick skin as you can get all sorts of ridiculous questions like is the coin real or wether you want to sell outside eBay (or the platform you choose). I just try to be polite when I decline or answer that the item is real but honestly, sometimes....[Patience is a must!]

A few things I noticed that successful sellers practice:
A) start at 0.99 or 1.99 or whatever low price but make it a true auction rather than start at what he/she thinks its worth
B) offer low & combined shipping (at first & then increase as you get to know exactly how much it will cost) to build up attention & create a following
C) offer new listings/sales of a variety of items weekly (what I found so challenging) to build up a following
D) if they offer Buy it Now (BIN) then they do this at the end of their auction listings.

I am not a seller, but a person looking to expand my personal collection.
Even though you have unwanted doubles, there are plenty of people on this site, such as myself, who could be looking for those coins for their own collection. That being said, you might want to consider placing them on this site to be swapped with other collectors. With enough swaps, you can make sales here as well (read the policy).
Just a thought/second opinion.