Don't buy solely because of that super low price. There are many considerations to be taken into account, one of which is the price. Do attempt to purchase captive-bred reptiles. Many of the reptiles available today are captive bred although wild-caught are still available. Do have the enclosure prepared before you bring home the reptile.
Stress causes many problems.
Eliminate the stress and you eliminate problems. You can freely redesign the enclosure and check the heating, lighting, shelters, etc. Do make sure you have supplied all the necessary accessories. The water bowl should be large enough to accommodate the snake if it wants to have a soak. In performing the examination and learning how you keep the reptile, your vet can often detect problems before they get serious.