So when is cheap stand up paddle board a good buy?
A cheap SUP board is a good buy when it's the right size and type for you.
A cheap SUP board is a good buy when it has no dings that need fixing.
A cheap SUP board is a good buy when it is a reputable brand and its the right size and there are no dings that need fixing.
What the big deal with dings? The way SUP boards are made, when they are dinged (damaged) they can literally suck water in when the temperature difference is greatest, i.e. when the board is first put in cold water after being in warm air. This can be disastrous and completely wreck your board if you let it take in too much water.
If you can afford to buy a brand new board, by brand new. Leave the used boards for people who just don't have the money for a new board. Buying brand new means you will be able to buy the right size and type of board without having to worry about it coming with hidden damage that you could suck in water.
What about cheap used inflatable stand up paddle boards? If it is a Red Paddle Co board, and it its the right size for you, it should be OK, but they are so good that very few people ever sell them. If you are lucky enough to find one, check it out, make sure it's the right size and if you are saving a significant amount, you better buy it, because if you don't the next person who looks at it probably will!
What about cheap new inflatable stand up paddle boards? You get what you pay for and they are cheap for a reason. Don't buy them!