I'm almost level 26
A few musings and tips:
The ultra-rare stuff seems pretty useless atm as I haven't had a single request from any animal for a football fish, jewel beetle, koi, etc. I figure the requests will come once an animal get above level 10 (my amenity for level 15 for the "cool" animals is crafting atm). So currently theres a bunch of these rare creatures at exorbitant prices clogging up peoples market boxes. I'm sandbagging all the rares I get in my inventory and not listing them for the time being.
The "medium rare" items (bass, snapper and horned dynastid) have been requested relatively often for me, so it's a good idea to list these. The animals give 1k bells for each of these when requested (plus +3 XP and crafting loot), so you can sell excess of these for 1k each or a little less for a more enticing sale.
To complete requests efficiently I've been keeping my stock of common items at around 5 apiece. So to supplement this I've been buying tons of incidental stuff from peoples Market Boxes when their avatar is hanging out at one of the resource gathering locations, so its a good idea to keep a variety of common stock listed so people like me can scoop it up!
I bought lots of underpriced stuff off Andrex yesterday, you need to charge more! I'll often pay several hundred bells for a common item i need if I need it to complete a request for an animal that's rights there.
Also I burned through all my calling cards and request tickets while grinding up friendship levels. Calling cards are a whopping
30 LEAF TICKETS each to purchase. Really pricey.
I made the sucker mistake of buying the KK Slider chair, turns out if you have that and the Tom Nook chair on your campsite, only one of them will appear at random. Terrible. Feel a bit ripped off by these premium items but the collector in me had to have them