MkVerstappen

I want to buy a mountainbike that I can use for trail riding "offroad". I know some cheap bikes have a warning that they are not designed for that
How can I decide which is a real mountainbike?
I want to buy a used one, probably hardtail. Is that ok? buying a used and then riding it. (I have 300$ limit)

2old

Depending on your local market you should be able to purchase a reliable bike for $300, especially considering the current market conditions. If possible, take a knowledgeable friend when you look at anything used. If that's not possible, find something from a reputable company (Trek, Giant, Specialized) that looks like it's in good condition cosmetically fits you comfortably, and rides, shifts and brakes adequately when you test it. You might go to a local shop and ride a few bikes for comparison. Also if you locate something, you can post here to see if it's worth the asking price (after you get ten posts).

veganbikes

I would try and raise your budget to get something more modern used. In the end you are probably going to need to invest money to fix it up. Look for Deore or above components and other stuff from known manufacturers of quality. Know that you can easily find a bike that needs a lot of work and maybe not notice. Also know that many front forks haven't seen maintenance as they should and that can get expensive or hard to get done as the fork is older and many companies may not want to work on it. You would be wise to see if your local shop is willing to evaluate the bike mechanically (not for price) and also see if the seller of the bike you like is willing to meet up there that way you know how much you will spend on top of buying the bike or know if it is worthwhile or not.

Spending $1200+ is a huge step up from your $300 but it will get you a new proper mountain bike you can actually ride and enjoy. With the new bike you have all the warranties and usually some service attached (at least one or two free tune ups/check overs) and in the end you will have something more modern so if you decide you want to put a few upgrades to it, you can find more options.

Plus with a new bike you can test ride a bunch of stuff more easily and get better expert advice that is harder with used bikes unless you have someone who is actually knowledgeable in bikes because they work in the industry or are a private bike nerd. The classic "bike friend" is utterly useless and is more a sounding board then someone who knows bikes, maybe they ride but probably have one or maybe two bikes and really know little some even commute everyday which usually means lack of maintenance and of course missing knowledge. There are a couple "bike friends" I have seen who are a little bit more knowledgeable but on a slight enthusiast level but still miss out on a lot.

prj71

$300 = Department Store Bike Shaped Object.

Trakhak

Yes, for an off-road beginner, a hardtail is OK. (Which reminds me: if you have friends who are experienced off-road cyclists who want you to ride with them, trust your instincts about speeds and terrain that seem dangerous. I've known of several new riders who tried to follow faster riders and ended up in the hospital. One had to have his jaw wired back together.)

There are some kinds of damage and wear to bike frames and components that would make buying a used bike a bad idea and would not be obvious to a novice buyer, but bikes that look very clean are usually a good bet - lots of people buy bikes with the best intentions and then soon give up riding.

Finally, post your height and identify the Craigslist and/or Facebook Marketplace that serves your location, and people here will be able to help you find a locally advertised bike that is likely to fit and to be appropriate for your intended riding.

Cdubs

I noticed in Canada GT Agressors are at a huge discount. So I cheked the states and it seems they are on sale too. At least at Dicks. IMO these are great beginner bikes and will allow you to upgrade a bit in the future.

https://www.dickssportinggoods.com/p...gt%20aggressor

SpedFast

https://www.walmart.com/ip/Ozark-Tra...RoC-ocQAvD_BwE
This Wally bike has gotten lots of good reviews on Utube. For a beginner, how can you go wrong? HTH

prj71