Haven't used LetsBuy, so can't comment.
However, I have done more than 15 transactions over 2 years with yebhi, earlier they were known as bigshoebazaar, and were also on ebay. They are by far the best online site I have ever dealt with. Their shipments reach in 2 days of order, and never have I got a seconds - touchwood. Once there was a problem with one of the shoes - a scratch, and I sent it back and got it exchanged. Customer care responds immediately on the phone and chat. Haven't tried the email option though.
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Folks mentioning about spamming from any of these sites - please also mention your purchase experience. End of the day its the purchase that matters more than how many SMSs they send.