SIM Not Valid for iphone 4s?

I bought an unlocked iphone 4s off of ebay (software is up to date). I bought a straight talk sim card, put the sim card in, and went through the activation process- but it still gives me the message "SIM Not Valid - please insert a compatible sim card"

the description of the phone on ebay specified that the phone s unlocked and compatible with any GSM carrier

PS i found a verizon sim card when i opened the phone…

It sounds like the phone isn't actually unlocked

I agree, the phone does not seem to be "unlocked", email your eBay seller

Could be that its not unlocked, or maybe sim card is incompatible, Verizon runs on CDMA network, while ATT, and Tmobile run on GSM network so the sim cards are not the same.