Depends how into it you want to get. If you just buy the first grotty looking mill you see you could slap paint on it and try and flog it. People buying used machines are often savvy though - I've taken to taking a DTI with me to buy stuff to check accuracy. If the ways of a machine are worn its going to need grinding or scraping, which ain't cheap. I tend not to buy metal machining stuff unless I know its going to sell well - most old imperial kit is getting to a point where its only of interest to farmers and hobbyists. Sheet metal fabrication stuff is much better - there's always a market for it. Restoring woodworking machinery is much easier as the wear tends to be less and there isn't the need for thousandth-accuracy ways. As for where to get it from - ebay, facebook, gumtree, local auctions, car boots, online industrial auctions and my favourite - word of mouth. Lots of people know I buy machines and give me leads on stuff - always pays to chat to people!