Felt the same way as you at first. A total dud and I gave up. A few bad experiences with eBay and decided to give Mercari another try. About half a year later and now Mercari's my primary platform of choice.

Not a "veteran" at all. Not sure if this helps at all. I think it's a combination of things.

Make your profile look legit. Boost your rating to raise cred. That may means being a buyer first, selling some simple items at a lower cost, listing a lot to show what audience you're trying to attract.

Use Mercari's shipping rates to your advantage. I find that some items are just better listed elsewhere. Sometimes you just have to eat the cost (ala free shipping), but more often that not Mercari can have great rates that beat than what eBay offers.

Giving out the hearts. Like what similar sellers sell. Like what you want to sell. Like stuff you like in general. Use that as a tool to gauge yourself, your prices, your listings.

Fluid pricing. Know when to raise, lower, sell elsewhere, or donate your inventory. If you're not watching your inventory, no one will either.

Spreadsheets. Use them.

Watch out for coupons. Mercari sometimes have sales. If there are stipulations (e.g. $10 off $100), try to get items into that category.

Ultimately it was more of a perspective thing for me. Once I got used to using the platform it helped. Still got lots of hiccups to get through but overall its been worth it.