Look at the reputation of the seller and usually if they specialize in only laptops that’s a good sign.
Additionally avoid really poorly made laptops, I usually avoid most HP laptops because they have the rigidity of a wet newspaper and break all the damn time. We get a lot of them for repair and it’s labour intensive and expensive. Some dells are just as bad so be careful.
Most of the time Thinkpads are pretty solid little machines and have pretty good build quality. Additionally if you want something to last it’s worth looking at business and professional machines, many consumer grade ones suck.
A specific seller I have bought from are “PC Server & Parts”. They have a free 90 day warranty and good customer service, their prices are also very competitive too. I would recommend looking at their website too (true for many eBay sellers) since the prices may be cheaper (eBay charges a fee to sellers).