Good Afternoon Rich,
In my opinion a reputable chip dealer or even a private sale for an amount over $500.00 should guarantee the sale for at least 30 days. However I would place the following condition for return:

The chip has not been removed from its original holder. I would a use cardoard holder stapled in four places and initialed over the staples.

I would maintain a scan of the chip of both sides in its holder so that it would be easier to prove if the chip was removed.

Finally, if I'm dealing with someone I know and trust I wouldn't worry about and just takle the chip back or refund the difference in trade or cash.

I treat collectors the same way I would want to be treated.

Jim