Can I use another companies iPhone 5 and switch it to Verizon?

My boyfriend is able to get me an iPhone 5 for 300 that his friend is selling buts its another company I think it's an AT&T not sure but I have Verizon could I activate it to my Verizon

No you can't… If you call verizon they will tell you the same thing… Matter fact they would probably laugh at you. So no… Any iphones sold for at&t could only be used on at&t & on T-mobile since it GSM… Verizon uses CDMA Technology.sorry

YES, you can
ignore the people that say "its GSM" and "its CDMA"
the iPhone 5 is a WORLD PHONE, and will work on any GSM or CDMA network

you need to get the phone unlocked, then you need to go to Verizon and ask for an LTE SIM card. Once you install it, it will be working with Verizon.

If it's AT&T, you can't use it on Verizon.AT&T is GSM and uses SIM while Verizon is CDMA and doesn't use a SIM. So your boyfriend will be wasting money. Ask him if he can get hold of Verizon iPhone 5.