If it has already traffic, there is no reason why it shouldn't continue.
Unless...
... the previous owner artificially generated traffic, then it might decline promptly.
... people come looking for content, and you don't update the website: I own some (non commercial) websites with content, if no new article is published for some time, I start to see a decline.
... the website has no backlinks - then, if it is not a keyword, it seems bound to decline (check with Alexa the sites linking in, this will help you to assess if it is likely that backlinks will continue to bring traffic).
There may be some other issues to be considered. Anyway, if a website has traffic, it is unlikely it will disappear overnight, but it may decline.
Keep also in mind that traffic does not always convert into clicks. In some cases, a parked domain will receive a high percentage of clicks (for instance, I have a domain name with only a dozen visitors a week, but 25% of them click on an ad; I have another domain with 1,000 visitors a week, and none clicks on ads!).