I am not surprised of your experience. Because this is a ZEV compliant car, Mercedes doesn't really want to make, sell, or support this car. The individual MB dealerships also hate this car because: 1) they can't make money on service, 2) they generally don't know anything about electric cars and don't want to know, and 3) this car is hard to sell because the public don't know about it.
In reality, the MB B EV is an excellent electric car, for the price, as long as you are not one of these: http://www.mybclasselectricdrive.com/forum/viewtopic.php?f=8&t=380
You might try these 4 things:
1) Bargain hard because there is virtually no demand for an used 2014 B. Try offering them $25k or less. I'll bet that independent dealer bought it at auction for under $20k. Then get it in writing that you can return the car to them if you don't like it for any reason within 90 days. The sudden loss-of-drive issue usually shows up early on, if you don't see it for 90 days then you are probably fine. Also keep in mind the car is still under its original factory warranty.
Consider showing that dealer this list: http://www.mybclasselectricdrive.com/forum/viewtopic.php?f=8&t=380
IMO, even if you get it for $19k ($25k -6k rebate), it is still better to lease one. If you can find one, a 2014 $49k msrp can be leased for $12,100 or less ($3000 + 35 x $260). So, minus $6000 from your state rebate means $6100 net to you for three years. That's a lot better than $19k out of your pocket today. $6100 for three years is crazy cheap for this car.
Here in California our rebate is $2500, so 12,100 - 2,500 = $9600. IMO $9.6k is still better than $19k out of ones pocket today because ....
EV technology, especially the battery, is improving quickly, they will get much better in 3 to 5 years, and no one will want these used EVs 5 years from now with a degraded battery. For these two reasons, no one should buy electric cars, only lease.
(Maybe it's okay to buy an used EV if it's super super cheap. For example, used 2013 LEAFs with original sticker of $30k are now selling for less than $10k. I won't even buy it for $10k, $7k maybe, because longer range LEAFs are coming in early 2016, no one will want the old ones six months from now.)
2) Another suggestion for you is to get that 2014 B inspected at a MB service center. Tell that independent car dealer it will be done at your expense and you take the car to the MB service center for the inspection. While at MB service center, ask the tech to look up the car's service history.
3) Since this is an used car, ask the dealer if you can test drive it for 24 or 48 hours. Take your time checking out the car and see if it stalls on you (or jumps to neutral while in drive).
4) See if you can lease it, for as cheap as possible, for 24 or 36 months. Check and see if your state will still give you the $6k rebate for leasing an used EV. Offer to lease it for $150 to $200 per month with zero down.
So, which state are you in where you get $6,000 rebate for purchasing an electric car?