General

Buy Nexon.

buy perm NX box and hope 4 the best i got a nox cape that i sold for 1.2b in bera

What the other person said.

Since there's no MTS anymore, you can't even buy items in there and sell. However, if it's on a card (which I'm doubting, because the amounts are 10k, 12k, 25k, 30k and 50k, as far as I know), you can sell the code for mesos.
But, if the NX is already on your account, you can gach, I suppose. Might get something good.

You can also sell items, in a sense. Someone can say 'if you buy me a regular store permit, I'll give you 90m'. So, they give you 90m, and then you gift them the store permit.
And make sure they pay mesos first. Never gift first. If they don't agree, don't trade with them.
^ Also, you might want to double-check whether Maple still allows this or not. When I last played, it was allowed. However, I haven't played for almost a year, and just recently returned (but someone asked me yesterday to sell them a regular store permit...? So, I think it's okay...)

You can buy meso bags, but DO NOT. xD The 3k one gives you 10m or something, when 3k should be worth more like 120m.

You can also try the cash shop surprises, as you might get some permanent wearable items, which can sell for 100m-1b, depending on what you get. However, it's more expensive to try than gacha.

Good luck

If you have it on a card, you can sell it for 1:50
If not, your best luck is with gatch right now. It has a ton of VIP weapons that you can save for when they're rare again.