If you stick to major retailers you should be ok. There is quite a lot of variance in price even amonst the big stores, but in general it's the usual 'buyer beware' adage: if the price looks too good to be true, it probably is.
I'd avoid all auction sites (you know the ones I mean) regardless of putative reputation of sellers, and while I can recommend Amazon.co.uk (good prices and they've never let me down) I am wary of using Amazon Marketplace for the same reasons.
Another worry-free route would be manufacturers' websites, which often have 'specials' and very resonable pricing.
Good luck 
Mick