The most difficult part is often getting all the docs required from the seller. If you can't get them, you haven't bought your girlfriend a bike, you've just given someone some money and borrowed their bike.
just to add some drama, 30 seconds after stepping out of my condo this morning, i was run over while walking along the footpath by one of those aforementioned "micro-penis manchildren in a pickup truck" - not sure if he was in an imaginary race or just braindead but i have three broken bones now. and hence i would like to add the point that despite all the problems i mentioned with regard to driving here, being a pedestrian is far more dangerous.
yes - potholes, loose pebbles/rocks on the road, slow drivers blocking the fast lane, suicidal moped riders cutting in front of you, micro-penis manchildren competing in imaginary races in their pickup trucks, other road users changing lanes without indicating, maybe a stray dog or two, possibly even a herd of cows crossing the road.
there's definitely a consensus among the foreigners who want to visit Thailand (and their Thai friends) but most Thais I know are still pretty worried about the virus (which makes sense with tens of millions of Thais still not fully vaccinated and hundreds dying with Covid every week).