which software is best for reparing STLs?

As in any other business, it is convenient to study the market, choose a target audience, calculate costs and benefits, develop a marketing campaign, and offer a competitive product in terms of quality and price, there are other factors, such as fashion, competition , trends, etc.
       And all this is just the beginning, you need a sales plan, a development plan, ideas are needed, not only in the creative field, but, especially in the commercial, because it is you who sells the product, the platform is only a necessary tool, but it is not decisive, hundreds or thousands of creators who compete with you in your field are supported on the same platform.

how do i know the market. research it myself? search keywords. see either that item is there or not. i am on tier 2 atm. so Statistics aren't open to me completely

Give it a few months then you can judge which of your jewelry are selling and which are not. Use that as a guide for your future designs? If they are all not meeting your sales expectation then you have do some market research on what the customers want.

your right..so far i have made jewelries with almost full gemstones. i have to try something without adding small gemstones. Experiment is the key i guess

Hey, KhatriCad, i'm very sorry but STOP MAKING NEW MODELS RIGHT NOW! Your models can't be used for 3d printing and is SUPER hard to work jeweler with them. First of all you need to know something about how processing model when it is already metal. Right now you need to spend time to learning or visit jeweller, then you need to remake your models and upload them again.
In other words, it's problem with how it will be printing and how jeweller will processing that model.
Anyway, you need about 300 models to make something like 50$ per month.

wait what? who are you? how do you know my models aren't good. i have worked with jewelers for more then 2 years now.no complains. all my stls are repaired and free of errors. recently i have done a jewelry product and he gave me 4 stars Good. you can check my profile

Also i have free files on my profile people have downloaded 100 times..no one complained it.

I viewed some of your stl in marmoviewer. They are bad, man. For example your ring
https://www.cgtrader.com/free-3d-print-models/jewelry/rings/indian-design-ring
- why it is SO fat? Too much of gold. Why prongs of main big stone are SO big and fat?
Next part is you don't understand how DLP printers work. Your ring will LOST those spheres under main stone, otherwise you need place support to each sphere - it is very hard to print with no errors with supports growing. And many of model pattern will be hard to polish, some CANT be polish.
I have worked with jewelers too, about 6 years. Make a design and operate 3d printer and printed models. And i have a lot of complains first time, when i learned.
That's why people don't buy your models now, because it's hard to work with them when it will be in metal, it's hard to even print some of them.
I just want to help you. You can use it, and you can call me asshole - this is your choice)

Look at this,
https://imgur.com/a/RS8lZHN
i fastly mark some problems inside of your ring
https://www.cgtrader.com/3d-print-models/jewelry/rings/flower-box-ring-with-beads
And how that ring will fit on a finger - will really rub finger while wearing
It is not all problems with that ring. You need always think how to wear you jewelry, how to print it, how to cut jeweller work time.

I don't want to argue with you guys, but jedi3d you must be professional, even if you're just going to give a feedback, you must differentiate between a constructive criticism and criticize, offend and ridicule someone's work.

Take into account jedi3d notes and and see if your new or fixed products they sell more.