Yes you can. At any Disney ticket booth. Also at Disney Stores, and there are some official Disney ticket sellers such as Ticket Mania and Undercover Tourist and AAA.
Tickets should not be purchased from private parties including via eBay, nor from roadside stands. Partially used tickets should never be purchased as all tickets are non-transferable once usage commences.
All current single and multi-day standard tickets (non-promotional; non-complimentary, non-annual, etc.) are in the Magic Your Way classification.
Like OP said do not buy from roadside locations offering these. Disney does not truly offer discounted tickets "per say" so why would they allow anyone else to. But locations such as Undercover Tourists and I am sure others offer a discount by certain means for example UC does not charge sales tax or shipping so it is discounted in that way. Do your homework and if it looks too good to be true, then it probably is.
Mickiethepooh is correct. SeashoreCM must have been distracted by the Red Sox or something.
The Disney Dining Plan is available ONLY as part of a package for guests staying at a DIsney owned-and-operated resort. It is absolutely NOT POSSIBLE to purchase the DDP if one is NOT staying at a WDW resort. Period.