Most mobile phones are 15 - 35% less expensive on eBay vs. BKK. PC's, and PC components are 20-30% less expensive on-line in the U.S. vs. Panthip.
If you want to check mobile Phone prices in Bangkok,
check the prices here, then subtract 15% if buying at a small shop in MBK. I would never , ever use a credit card in MBK (other than at one of the anchor stores) or Panthip, as it is too risky and requires an uplift of ~ 3%.
Personally I would not recommend purchasing a second hand phone a MBK, unless it belonged to you previously
. Mobile phone prices in the hundreds of small shops are all within 2% on any given model.
Sony PSP pro stick duos (Sony/Sandisk) are ~4,000 THB for 1 GB and ~ 8,000 THB for 2 GB, stand-alone. Prices are slightly less in a package including a PSP. MBK is the best place to buy PSP's. I can recommend a specific shop via PM.