It is just about as safe to buy used hot tubs, as it is any other piece of used equipment. Naturally, you are going to use caution if you are thinking of buying the item from someone you do not know. If you are not familiar with hot tubs then it would probably be a good idea to maybe visit a few hot tub dealers and get a general idea of what types of things can go wrong with them.
Aside from the working condition of the hot tub, the aesthetics of it is important as well. You do not want a luxurious item such as this that is badly stained or chipped. A hot tub should enhance the area where you are planning to install it.
Hot tub spas are a bulky and heavy items which cannot easily be moved unless you are buying a portable hot tub .You may want to ask if the sale price includes delivery. If it does not, you need to be sure you have some means of transporting it yourself. If you have to hire someone to do this for you then get a fixed price first as this can be a costly endeavor as well.
Another consideration you need to give some thought to is whether or not the tub comes with a hot tub cover. If so is this also included in the price? If not the price they are asking for it may not be such a good deal. Take your time and shop around and you will eventually find a good deal.