i'd say know your prices, if you're buying a low-mid range guitar, like something that's made in china, and you're living in london, get it imported from germany, you'll save a heap, in the US everything's pretty cheap so your fine there. If it's a more expensive guitar you might want to get it from a store because it'll be set up and prices probably won't be so variable.

If you don't like a salesman or if he's acting snobbish like he knows what you want, don't buy from him, don't even bother, because if you're uncomfortable and you're being pushed into something you're not going to feel right about it in the end. you have to make your own decision.

Tell the salesman your skill level and how long you've been playing, so you feel more comfortable about trying the guitars out and you won't feel like you're trying to show off or something.