I doubt there's a specific percentage that 'quarantines' sellers, as it were, since someone with only two sales resulting in one positive and one negative would be at 50% and it would be a pretty unfair system if it was automatically based on percentage. 

I think it comes more down to other factors, which would probably include things like dispute percentage and frequency, possibly seller non-performance reports, or other reports to trust and safety. A couple of years ago I purchased from a a relatively new seller who turned out to be dropshipping, and after a couple of weeks it became very obvious they had been let down by the company in a big way. They went from 100% FB down to about 60% before they were NARU, and it took a couple of hundred negs to get them down that low. My guess is PayPal disputes and negs were the only factors involved in that case (in other words, I presume that as the average sale price was under $10, most people just disputed and negged rather than taking the matter any further).

Obviously, I don't know what it takes for certain, as eBay never share that kind of information, and while some obvious scammers are dealt with rather quickly, others seem more resiliant.