sure a small print run is easily done at home:
the question is, if it is inkjet, it wont be rain-resistant. Important for posters that will be exposed to the elements. It used to be a problem only in the winter, ha, ha.
Laser is not so affected but professional dye printing is pretty much immune to water ingress. Obviously not a problem when used indoors.
Professional printing probably costs not that much more than home printing, the average real cost of producing full page full colour prints is still high if you do a lot of them (heavy weight or coated paper costs a fortune, even at trade), and changing/buying inks can cause heart-attacks. Just think about changing all 4 colour cartridges in a laser printer! You’d be lucky to have change from £100.
For volume printing, even 50+, pro printing is almost certainly cheaper. And there might be beneficial options available, double sided, gloss finish, edging/ cutting, etc.
Delivery costs, or having to go out to the print shop could be disadvantageous though: it’s not as convenient as doing it at home.
And some printers dont like short runs – we are more flexible.
Regarding laminating, the other common water-proof method.
I prefer the idea of water resistant printing, then affixing the poster to heavy (recycled?) cardboard to make stiff – really dont like lamination .. To use lamination for eco/ green/ environment organisations work seems so ironic to me, but we’ve done it as we can do 300mm width*, but it doesnt seem quite right.
Another alternative is a reusable plastic sleeve, taped closed.
*(specialty lamination to 210mm width only: banner, adhesive, magnetic, etc.)