I'm planning on buying a used snowboard and bindings, the issue I'm having is I have smaller feet (ladies size 6), and the majority of used boots are bigger than my feet.
Do I need "snowboarding boots" (I have boots that look very similar to snowboarding boots but they're not specifically for snowboarding)?
Would bindings that were used for a larger boot for example ladies size 10 fit a ladies size 6 or would I need a different binding size?
Also, what size board would you recommend I'm 5'5'' and 140lbs?
For decent performance, you will need 'snowboarding boots'. They are designed to be stiffer so that they can transfer the movement of your legs/feet through the bindings and onto the board. All bindings are adjustable and small ladies bindings should fit your boot size. You want a secure fit for the boot in the binding so that the sole of the boot stays stationary when wiggling your leg.
I would strongly recommend getting new boots and trying on many pairs to see what fits best. You could easily end up with a poorly fitting used boot that would give you blisters or make for a really uncomfortable experience. Getting a used board/bindings is fine as long as everything looks to be in good working shape.
As far as a board size if you are just beginning smaller is best, say 130cm. As you transition to faster speeds and bigger slopes you could size up to around 145cm. As a general measure, people often get a board that comes up to about the height of your mouth.
enjoy your time on the snow!