Its marketing only.

If its faulty you get full postage refund

If its change of mind they are not obliged to refund.

If it genuinely was worth $30, you wouldnt sell it if it cost $500 to ship, as the profit vs $ at risk factor is too high. These days with ebays buyer return policy you always risk loosing initial shipping cost as a minimum if buyer raised not as described claim

Hence the only reason is to catch your eye and make you look at the listing. Often these stores have the same item properly priced. If you then buy the reasonably priced one, the dodgy one worked by grabbing your attention.