There are several reasons but mainly are:
1. Changed their minds after do more researches or got better offers
2. Testing price see lowest what price the seller can let it goes
3. Make seller fed up then have to sell his/her item with cheaper price since it's schycological game
4. Wanna get Seller details for some purposes

Solutions for seller in those cases:
1. Asking the LY ID of the potential buyers (if receive offer by sms/phone call)
2. Ignore them if they dont provide info
3. If they provided but fail to proceed the trade then blacklist them by posting their info or report o blacklist FFK corner
4. Check the ID carefully see he has good trade record history or not..the more trade he has, the better chance for u to close the deal...
5. My personal experience: Simply asking info by sms, i dont bother to reply...if u serious to deal, why save 15-20 cents for calling...