On the contrary, common sense is the basis for much of the law governing the liability of people for harm done to others. Generally speaking, in a private sale, the seller has no obligation other than not to be dishonest about what he is selling. Anyone with the requisite cash can buy a Ferrari the day after they pass their driving test, or a frisky hunter or racehorse the day after their first ride on a donkey. It is possible for a person to assume a duty of care to another, but simply offering some words of advice about the item purchased are unlikely to result in liability if the purchaser uses it to kill himself. Indeed, saying something like "this must be only be used with proper training and in accordance with recognised procedures" emphasises to the purchaser that he proceeds at his own risk.
Even a commercial seller's obligations are in general concerned with the quality of the item sold: he promises that it will do what it is supposed to do, if used properly. Otherwise, you would not find many shops selling, for example, scuba gear or rock climbing kit (although anyone who has ever bought stuff in these shops will be familiar with the irritating "have you used one of these before?" approach of the 19 year old sun-tanned dudes behind the counter).