I've been living in Puerto Plata for 9 years; I've bought 2 used SUVs, successfully, in this country; my suggestions:
1. Used the 'Listin Diario' classifieds, paper & on-line; usually realistic prices; ocassionally you can find a bargain. "Motivated seller"!
2. Moca & La Vega have reputations for better selection & lower prices; I shopped in La Vega; many dealers; prices? I didn't buy there.
3. Try Rockys' Blues Bar, Sosua; (809) 571-2971;
marco@rockysbar.com; the American owner, Mark, has a working relationship with some Moca dealer
; with his cooperation you can take the unpurchased car for 1 day, bring it to YOUR mechanic, have it inspected, bring it back, & begin negotioations.
4. Avoid the small towns & small cities; high prices, small selection.
5. If your skin is white, duck! You never get the DR price. Try to work around this.
6. Spread the word in local bars, restaurants, colmados, & car dealerships; tell everyone what you're looking for, & that you have CASH!
7. Take your time. This can be educational. Enjoy yourself! Buying a car, new or used, can be pleasurable, here or in the DR! Good luck