i've done this a lot of times. my returns have been great thus far. i have a pretty good value gauge. some of my recent flips:
shirt for $6 -> $43
record for $2.99 -> $48
walkman for $0.99 -> $13 (less than expected)
AC line enhancer for $3.50 -> $114
toy keyboard for $4.98 -> $154
shirt for $0.99 -> $45
shirt for $0.25 -> $18
shirt for $1.50 -> $36
(these are all with shipping added)
so, i'm doing alright.
edit: about to list a NWT lacoste polo i got for $4.
My sister told me she once bought a whole bunch of nice clothes at a thrift store bag sale. All the clothes you could stuff in a paper grocery sack was $5. She hit the kids section and bought a whole bunch of expensive kids clothes, and then listed them on ebay for a tidy profit.
-Jay