I bought a G2X for my brother and I tested it for quite awhile. Its a great phone. In short if you can trust what the person says and are willing to take the risk, go for it.
Here are the pros:
- G2X has a dual core tegra 2 processor 1 GHz (very fast) vs the Mytouch 4g's single core snapdragon 1 GHz processor (good but not future proof)
- G2X has an 8 megapixel camera vs the Mytouch 4g's 5 megapixel camera
- G2X has a 4.0 inch screen vs the Mytouch 4g's 3.8 inch screen
- G2X has an HDMI port, the Mytouch 4g does not
- Slightly less battery life, due to the G2X having a monster dual core processor
But battery life is relative, it depends if you will keep vanilla android or install a custom rom like cyanogen or miui, you could see your battery life improve greatly by rooting and using a custom rom while using root only apps like Setcpu which configure your processor to save battery while the phone is idle.
Long story short, if the phone is in good condition, do it and goodluck!