There's "Amateur" and then there's "I pumped this out in about an hour."
The DM's Guild is already a pretty crowded marketplace. If you take pride in your work, even as a non-professional playing in WotC's playground, it'll be worth the money someone paid for it, worth someone's effort to read and use in their games. It'll be worth people finding and paying for and downloading.
If you just write something that sounds neat to you and charge $3.00 for it, that's...not necessarily worth the money or effort. I mean, maybe it's great, but chances are it could be better.
Some aspects of professionalism are slightly beyond the reach of amateurs. For instance, I'm whipping up a monster document right now, and I've got no idea how I'd go about getting art for it, and very little money to pay for it, even if I could. I probably couldn't afford a pro editor (even at a fan discount.
). But basic editing - checking spelling and grammar and consistency with how 5e presents rules and balancing it and maybe doing a quick playtest in Fantasy Grounds - that's something that just requires my time. And since I intend to ask for someone's money, it's worth doing.
It's also worth noting that smaller, shorter things are often easier to check in this respect. You could probably put together a good 1-page subclass and do the editing and such in a solid 48 hour period, and stick it up there for $1, and it'd be great even if it's got some errors. But a 12-page adventure would be harder, and a 20-page compendium of monsters is going to be even harder.
A good question for any amateur is:
what is the minimum viable product? Make that small product the best you possibly can. Think about what makes your offering distinctive. Spend a week on it. There's no real rush.