So, after beating Pokemon Moon's campaign, I'm trying to finish the local pokedex before the Pokebank is active and I transfer everything competitive I have. Because it's very convenient, I'm using the GTS trading system to fill the pokemon I don't have or I'm too lazy to actually hunt down. But everytime I go there I find the same kind of problems: A goomy in exchange of a Solgaleo or a magikarp LEVEL 90-100! Why? Why do people do that? They're not even shyny! I'm sure they don't expect to actually trade a caterpie for a legendary, so why do people do that?

Plus, where can I find megastones in the game, and where can I grind against trainers besides the league? I need to evolve mons and get money to buy clothes/TMs.

Get a Bagon. Bagons will be traded for nearly anything. I literally hatch a bagon, put on GTS, and its gone by the time the next one hatches.

Honestly I have no clue. I get trading legendary for legendary but I was looking to get a female Sneasel early on my game and most people were asking for Solgaleos and such. Good thing this gen you are able to filter out people who ask for that kind of trade.

GTS is really annoying

I reallly want them to add a feature where you can search for pokemon You are offering..
Have a spare Charmander? Then see who wants it, and what they are willing to trade

Really annoying sifting through pages and pages to find a ditto that is not being traded for a legendary

Finish the Looker quest for $1m.

IDK about grinding, though.

Those people are annoying, but there is the option to only display people who want Pokemon you have. I completed the Pokedex through GTS pretty easily thanks to it. (Trading Pheromosa for Cranidos did feel wrong though, even if they do give you four of them)

1. when selecting a pokemon to find in the GTS go to options and you can hide all trades that request legendary pokemon. it narrows the results down quite a bit and from there its easy to find the people who are also using the GTS for pokedex completion. i was able to get quite a few of them and i now only need 5 more for the Alola Dex, which i will get tomorrow once my friend can do trade evolutions with me.

2. Mega Stones are only sold at the Battle Tree using BP. kind of a pain really, but its the only way

3. Poni Gauntlet is perfect for end game grinding. i have been able to level up at least a dozen pokemon to the mid 50's by grinding there with the Lucky Egg that Kukui gives you after you beat the elite four. the pokemon there are all above Lv.50 and there is one trainer who fights you once per day. admittedly the elite four is probably quicker, but Poni Gauntlet feels a lot less tedious

Why is that a bad thing? You know many great people traded me good pokemon I didnt want to wait to get? In Pokemon Black I had my full team the minute GTS was available.