I started selling at conventions around 2017, more seriously in 2018 but dropped off around covid and am sort of getting back into it again. I've only really sold annually at two local anime conventions in my state (I live in the midwest USA) one was small, and the other is a very huge anime convention that actually has a craft fair for the folks that couldn't get into the actual artists alley. I don't like to sell much at anime conventions because I prefer to attend them, actually. (cosplaying and hanging out basically, its like a vacation for me :'D Also artist alley hours are so long!! )
When I did sell at anime conventions, though, I didn't have a whole lot of variety, but I'd basically sell some original stuff (my comics and original t-shirt designs) and then fanart merch (stickers, charms and pins). I'd usually break even on the table and have a decent profit left over, so it wasn't too bad!
As for other artists, I never had any rivalry/drama issues, the ones who did venture from their tables were friendly and I definitely used to network there when I had the chance! It's especially nice in local circles since if you go a few years, you get to know all the local artists and I've made a lot of long time friends from selling. It's really sad to hear though that in some conventions artists interactions are not very friendly!
Because I don't do a whole lot of fanart, the anime convention crowd was hard nut for me to crack, and the merchandise I did have was just not trending or relevant anymore. (I heard a lot of, “Ohh I remember x-series! I used to love that!” from attendants) These days I prefer to sell at craft fairs, zine fests or small press comic cons. There's always a variety in places like these and I prefer to make merchandise that can appeal to different age demographics and non-comic readers. (Small but useful stuff like greeting cards, memo pads, etc) My best selling items in person, which surprised me at first, are t shirts! I can't sell them very well online but they do well in person. That usually helps my table even out, and if I can try, I can get a few people to buy my comics. (I usually like to sell short and cheap $5 zines in addition to my main comic, which is $25.) That's just how I like to come up with merchandise—but there's a whole lot of ways I've seen other artists design their own items, stuff like earrings, zipper bags, tumblers, notebooks etc. The idea is really just to put your art on something useful. Selling is so hard!! I'm also very short so I like to stand up when I sell, which can be very draining haha. But I do notice that saying a quick “hi” to everyone who walks by helps out a lot. When I was at the anime conventions though, there were some people who would literally shy away and leave if you said hi, so sometimes it's hard to gauge. :'D I guess that's all I really have for my experience...selling is hard!! I wish I had the energy to do more, I only do about 2 or 3 events a year, mostly due to transportation limitations.