Now go look at the table
Feel of the rails, all of them, if any are hard or mushy you will need to replace them all. Usually you will have to recover at the same time so factor that in. Are all the parts there. Look under the rails and make sure all the rail bolts are present. If not some may be broken off and cant be replaced. Is the table particle board, depending on the table this may not be a problem but it affects the price a LOT. MAKE SURE it is a slate table. If not walk away.
Ask about accessories,
what comes with the table, if there is a light is it includes. Cheap accessories are not worth anything at the negotiation stage, they are no use to the seller and may not be to the buyer either.
Try to find out how motivated the seller is and how long the table has been on the market.