I recently purchases a blackberry off ebay and when I insert my SIM card its says invalid SIM, I tried inserting another SIM and I got the same message. The phone is currently locked to T-Mobile, but the SIM I use is T-mobile. Please can I get aome advice on what to do.

Thanks

Drop by Tmobile and get a SIM swap for new SIM and see what happens.

Is the device Tmobile USA, or Tmobile Europe?

ON the homescreen of the BlackBerry, press and hold down the lower left ALT key and type E A C E. There is no box to type this and you won't see your typing, just type those letters.

What is the vendor ID noted there?

I have already went into a t-mobile store today, and they tried that but still the same issue, they said the phone may have to be sent off for repairs, but as it is not the phone linked to my contract then they were unable to send it off for me.
The phone is T-Mobile europe. the vendor I.D is 100

The phone is not Tmobile DE, UK, AU, PO or CZ, HU, etc., it is Tmobile USofA.

RIM Vendor Codes - BlackBerryFAQ

Get it unlocked and you'll be fine.

How do I unlock my BlackBerry? - BlackBerryFAQ

Oh, it all makes sense, thats why the settings are different to my old blackberry, thank you so much for that you really made my day, can I unlock it myself or do i need to go to a store an get it done?

You didn't read the link up there did you?

Being a used devices off eBay, I sorta doubt your Tmobile store will unlock it.
You can unlock it yourself, once you buy an unlock code. Try Horizon Wireless, I trust them.

no i didn't lol.
Thanks soo much again for your advice, took it to a shop and they unlocked it for me and it is working perfectly

Great, very nice of them to do so... the unlocking policy varies from carrier to carrier, you are lucky they did so for a device they didn't sell you.