Hi I'm looking to buy a new acoustic guitar, on a budget so preferably second hand, I've been looking at eBay and guitarspost.com and they both seem to be pretty good, I think guitarspost is pretty new though, basically is it a good idea to buy guitars online, or is it better in a store??

It's better in a store because you get to actually play what you're buying so you know for sure that you're making the right decision, but realistically that's not always possible.

I've bought & sold at least 50 guitars on ebay over the years and never had a problem.

Your loca craigslist is also worth a look.
Gibson LP Traditional, LP GT, LP Studio, SG Standard x2
Barber Tone Press > EHX Worm >TC Polytune > MXR Custom Badass 78 > EXH Glove > EHX East River Drive > Zoom G3 > TC Spark Mini Booster
Laney VC30
Marshall TSL602
Jet City JCA22H
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Wow - is this your 1st post?

With an acoustic, especially, you should look to try before you buy; the classifieds here are a possibility (coincidentally, I've just posted one!).

Of course, research the prices before committing - check ebay going rates, gumtree and suchlike.