Unless you have knowledge anyone who takes the sellers word that it will pass an MOT is naive, especially if they are paying full MOT prices.
I usually buy without a vetting, but I factor how much if I would lose should it have a problem. I buy budget cars with a full MOT privately, but expect to spend at least £500 sorting out anything that's likely to be wrong with it, I am not a mechanic, and it would have to be sorted at the local garage. The MOT just makes it legal to tax, insure and drive it to the garage, as long as it's safe to drive on the road. At the end of their life with me they are sold as spares or repair, with one month's MOT, so they can drive it to a garage or home if they want.