You may not of thought about this, but probably the most important thing you need a caravan mover to do is to stop and hold a caravan on a slope. That depends on the movers ability to grip the surface on which its operating. The maximum grip an a frame mover can achieve is dependant on the force that pushes it to the ground. Generally this will be the equivalent to the nose load of the caravan or about the same as an Adult. and that is a very small proportion of the weight of a caravan.
If you are thinking of using the mover on a slope, consider the fact that if you couldn't personally stop and hold the caravan still on that slope without using the caravans brake, there is a very real probability the A frame mover could not manage it either. The problem becomes worse if the ground is slimy from leaves, rain or snow and ice.
If the ground is lumpy (like some caravan sites) the mover is unlikely to manage to drive the caravan over the bumps.
Their effective ness will also depend on which way the caravan faces on the slope, If the hitch is up hill, the force holding it to the ground is reduced which further reduces its grip.
One of the reasons so many of these are for sale really cheap is they are not effective except on hard flat ground, and if used on slopes can be highly dangerous both to people and property.
I strongly advise you NOT TO BUY one of these A frame movers.