Shared proxies probably no, but used proxies probably yes. If you buy dedicated private proxies, you'll be the only one who use them until you pay for them and they weren't used before on the site you're buying them for, but they could have been used on other sites. At least this is true for reputable providers. If you buy Twitter proxies, then (more than likely) they weren't used on Twitter before, but they could have been used for Google scraping for instance, hence when you try these supposed to be virgin proxies, most of them already banned on Google, but they still can be good for other purposes you're buying them for.
