Reliable at the price range of about $100 US is problematic. You're just going to have to get lucky, as most used saws for $100 are going to be real old. If it were me, I'd look for an Echo if they are common in your parts. Your bang for the buck will be higher and the vintage could provide a saw not as old. Someone mentioned Stihl 009-011. I've had two and reliable, they were not. Because they were old when I bought them. I do realize that others like those saws; I just have not had good luck. Another thought is... around here, there are countless Stihl 250s for sale on Craigslist and Facebook Marketplace. Most people want too much. In fact, I don't recall seeing anybody selling one for $100. But there are so many, there is no way all those saws are selling. You could just make $100 offers to those sellers and, eventually, someone might get tired of owning it.
Now that I've written all that, methinks your budget is too low to have a reasonable expectation to find something reliable. Once again, you will have to get lucky. Double your budget and you could easily find a Stihl 026. Which, to me, would be a better choice than a 250. And if you double your budget, you could find a BRAND NEW Echo 35cc cs352 for sale. I saw two yesterday from two different sellers, brand new, in the box, and both were $175 US. That's a good deal.