Assuming the seller says they don't know how to tighten them, or is being economical with truth.
I take it you mean all the legs slip. In which case remove the handwheels, check the threads to see if they have
been partly stripped, if so you will have to source some handwheels from a tackle shop, and tap the thread out in the leg block.ask at tackle shop what thread they are, it will be a metric thread.A cheap tap set will do the job.
If they are not stripped, try and see if another brand of box leg will fit, if your box has square legs, PI do them in various sizes,ie 19mm, 22mm, if they are round, most major brands will fit, ie 25mm, 30mm, I am not sure if sensas do a 36mm dia leg,
You never said if it is the inners that slip, or the main leg in the leg blocks. If inners just get a set of 4 replacement legs