Assume it is original. This goes against what people are probably preaching, but full sized, double sided reproductions are not cheap to produce and unless a box office smash or a cult classic, not worth the time or effort to reproduce.
Find sellers you can trust, and stick with them. Unfortunately, 90% of ebay sellers absolutely cannot get a poster from point A to B in good shape. There are titles that are tough to come by for the smaller film companies that you may have to roll the dice on, but major studio releases, stick to those you trust or prepare to be disappointed over and over again. In addition, over grading is a serious problem on ebay.
Dont buy inserts (14x36) from the 80's unless you buy from Bruce, Dave, Kirby, Peter, Rich or one of the other guys that have been around a long time and have had enough of these to know the tells on these.
Reproductions normally can be spotted quickly, wrong paper, size, printed on large format so paper has a texture, pixelated image etc.
Dont buy French posters unless you buy from someone that knows what they are doing. TONS of rerleased stuff, often advertised as original release and incorrect.
Single sided posters are still printed, that does not mean they are fake. Canada still does a lot of single sided stuff, normally 27x39 though in size.
Rolled posters exist for most titles post 80s and likewise folded still existed into the 90's. Pre 88 rolled will bring a premium, likewise post late 80s folded will take a major value hit.
Dealer sites, prices are retail, some even over inflated, but there are costs involved with running these operations, so dont be shamed from buying from them. You just need to be comfortable with the price regardless of the poster police telling you what a value should be.
learnaboutmovieposter.com is your friend, still hard to navigate, but useful information for beginners and quite a bit of back story on the hobby as well.
Reference the bad seller list on the board, there are some you absolutely should not buy from, period. Some 50/50, personal judgement, but a useful tool to reference.
Have patience, unless you are a universal horror collector, posters will come up for sale again, so dont be afraid to walk away from a bidding war.