you need a dealer in order to buy and sell vehicles without titling and registering them in between. hence dealer plates.
personally i would probably sell a bike with an open title to help someone out but would never buy one. You have no proof that the seller has the right to sell.
Every bike or car ive ever bought (which is like 20) i have always had 4 or 5 things
sellers id, compare it to title, compare vin on title to vin on bike, issue casheirs check to name on title, draft bill of sale referencing all of the above
if they wont take a cashiers check i walk, if there name isnt on title i walk
if it was easy to do what OP is askign everyone with cash would be in the car flipping business. But title, tags, taxes, and insurance caught into profit to much. you cant avoid these things legally....especially if you plan on riding them.