bike size

Hey I'm looking to get this used bike n I was wondering if its the right size for me. The owner says its 50cm**oh yeah I'm 5'7..5'8 on good day

That's most likely too small for you. Most guys 5'-7" and 5'-8" do well with frames sized 54 to 56cm.
It's usually safer to go towards the smaller side of the range as one can usually fine tune the fit with adjustments to the seatpost and stem. Otherwise, if the frame is too big, there would be nothing to adjust it to fit you.
Chombi

50cm is about the smallest bike available with "normal" wheels, suitable for someone 5' tp 5' 4", depending on leg length. You should be shopping for something around 54 to 58cm.

Thanks for the feedback

it's important to know what you're measuring, whether it be the seatpost (center to center) or something else. i'm almost 5'11" with a 32" inseam and ride 22" (56cm) bikes measured by the seat tube c-c. this provides 4" of seatpost exposed, which looks rather perfect. for those 5'8", look for a 52cm bike measured c-c or maybe something slightly longer when measured c-top.

I'm checking the bike out tomarrow to see for my self...but what's your take on me rocking a 57cm

I am 5'11" and ride a 57. I would put you on a 52/54

6'-1" tall and a 31" inseam, I ride a 56cm Cannondale. FWIW, really short legs and a long torso.

5'8" (and half if you wanna be real accurate), 31.5" inseam. My fave frame size is 56 c-t-c. Started out on 52, but that was way too small, ended up with huge saddle to bar drop, just wasnt / couldn't get comfortable, Id have the same issue on a 54, way too much seat post showing IMO and can't get bars high enough, or it looks terrible with all the spacers under the stem. 55/56 is key for me, with a slightly shorter than "normal" stem.

I agree that 50 cm is most probably way too small for you. Just a (somewhat) dissenting viewpoint here: which is, don't be afraid of a larger-sized frame if it makes sense for the important reasons. I'm 5'10, with 32" pants inseam. I prefer a taller saddle height than most people (78 cm from BB centerline, 94 cm from top of pedal), and I also like to have my bars at least as high as the top of saddle. I find that this is difficult to do with a frame in the 56 to 59 cm CtT seat tube size that most people my height seem to favor, because I end up with a lot of seatpost exposed (I know that's been the fashion for a long time), but worse yet, a tremendous stem column exposed to get the bars that high. A frame with an up-sloping top tube would lessen the latter problem, but I'm opposed to that on grounds of my C&V religion. So, I ride frames up to 63 cm tall, basically to get the top of the head tube where it needs to be. If you prefer the bars below, or well below, the top of saddle, then a smaller frame makes sense. Lots of people make a big production of their concerns about injury to their precious reproductive organs with larger frames; I think that's a load of crap. I can't "technically" straddle a 63 cm frame with 35 mm tires standing straight up, but I never do that when stopped with one foot planted: the bike is always canted, and this feels completely natural. Whenever I've had an accident, it's never of the "crotch falling onto the top tube" variety, but toppling sideways or going over the bars, or ending up with the bike on top of me.

Saint Sheldon (Brown) and others have made the case that top tube length is more important than seat tube for a good frame fit. Most of the bikes I like to ride have a 57 cm top tube, and it's fine with the right sort of stem reach, and there's also saddle fore-aft adjustment too, for me anyway. So, personally, I find that the top of head tube is more critical to me than tuning the top tube. It used to be that older British bikes, particularly, had top tubes about 57 cm or shorter, even for sizes with seat tubes up to 63 cm; more modern "proportional" sizing has a longer top tube for larger sizes, and the converse. Unfortunately, unlike seat tube (which can be ballparked by standover or a short ride with some saddle adjustment), it's very difficult to know what your top tube "sweet spot" is without having a bike that you've gotten to fit well already, that's comfortable for longer rides.

Probably too small, BUT:

I sold a 50cm bike to a guy 5-10 this week. When he called, I told him, the bike was way too small for him. But he shows up anyway, and guess what, it was a nice fit for him.

A couple of years ago, I had a 63cm bike that a guy 5-11 wanted to buy. I told him not to bother, that the bike was WAY too big for him. Well, he wanted it, and drove 80 miles one way to see it. And guess what, it fit!

While there are common beliefs on sizing, everyone is built differently, you could have short legs for your height, or long legs, or whatever. Sizing someone via a forum like this one: impossible.

While we can all share our own sizing information, and provide general rules of thumb, ultimately, the fit is yours to decide.

I know your pain. 5'11" and a 29" inseam (pants size).

Me too, bicycle inseam is slightly under 30. So I give up standover completely, its often negative on my bikes, to get adequate TT length. I ride anything from a 55cm to a 57cm usually. Just picked up a larger MASI, I am going to give it a try, as it was just too good of a deal, and I am not ready to see it depart yet.

fit is really personal. i ride a bike now that is a full 10 cm bigger than i used too! it all depends on how you like your bikes to feel i guess. i like to baaarely clear the top tube with my feet flat on the ground. that way i can use a regular stem and show only a fist of seatpost and still be super comfortable. some old french constructeurs built their bikes where barely any seatpost at all shows!

anyway recently i've figured out just how long my legs are and they are looooong! i can stand next to people that are a couple inches taller than me and our legs will be the same length.