The other problem with buying a bike online is that the bike will probably come only part-assembled & if you don't know what you're doing you can really make a mess of things. A proper bike shop will be doing this assembly for you: it's one of the things you're implicitly paying for when you buy a bike from them. Decent bike shops will almost always offer a free "tune-up" service which you will otherwise need to do yourself. Again, if you know what you're doing and are happy to do it yourself then great: go for it. If you don't however then you're likely to end up with a bike that just doesn't ride right after a month or so & you won't know why.
(A UK consumer advocate program once bought a bunch of cheap bikes online: universally they were either poorly put together or in some cases actively unsafe. One bike had the forks on backwards!)
(A UK consumer advocate program once bought a bunch of cheap bikes online: universally they were either poorly put together or in some cases actively unsafe. One bike had the forks on backwards!)