The construction of the top is fundamentally wrong & will never survive long term. That is because the solid wood slats will expand & contract a lot across the grain with changes in humidity, whereas the plywood or particle board substrate will not.
That aside, it's now yours & you might as well try to salvage it. Repair is complicated by the fact that new glue generally doesn't stick well to old glue. The best bet would be to remove the top from the iron frame & work some epoxy under any loose slats & then clamp the heck out of it to force things flat. Make sure you use slow setting epoxy, not the 5 minute stuff (you'll thank me later ) Make sure you use pieces of wood on top to protect the slats from clamp damage. It would help to rough up the old glue surfaces to aid with adhesion. To do this, you could stick some coarse, say 40 or 60 grit, sandpaper to a putty knife with double sided tape & work that back & forth under the slats.
The aesthetics of rustic furniture are personal taste, but the sad thing that is almost universal with it is that it is built by people who don't know and/or don't care about good building practices or how wood moves.
You've got nothing to loose by trying & good luck with it. And the seller is just a dishonest jerk & deserve the most negative feedback.