I'd recommend looking at Vintage sellers with lots of positive feedback, then check the 'Sellers To Avoid' thread on this forum (http://starwarsforum.co.uk/viewforum.php?f=24) to make sure they haven't given bad service to any members. After that, you should hopefully be OK. Given the huge number of people selling Vintage on eBay these days it's a little tricky to recommend one seller over another. My own experience is that most sellers are ultimately honest when it comes to whether weapons are original or repros, but the buyer, as ever, needs to beware. Make sure you know your stuff and use reference guides like the Imperial Gunnery to check pictures of weapons (http://www.imperialgunnery.com/). If you buy without really knowing your subject, you are asking for trouble as there are some rotten apples out there, and of course some honest, but ignorant sellers too.
Also, it should go without saying that you should keep a look out on the 'Star Wars Items For Sale' thread on this forum (http://starwarsforum.co.uk/viewforum.php?f=4), as people selling stuff on here should certainly be trustworthy, if they have been a member for a while, and especially if they have good feedback in the 'Feedback' thread (http://starwarsforum.co.uk/viewforum.php?f=22). Brand new sellers here will hopefully be trustworthy too (everyone has to start somewhere, after all), but exercise caution and common sense and use PayPal goods payment rather than sending money as a Friends and Family payment.
I hope that helps
Good luck!