Hope this isn't a stupid questioin.

If I have 15 widgets and I sell the whole pile at once, does it have the same impact on my Speechcraft skill as 15 separate transactions?

If not, is there any keyboard tomfoolery I can use to sell large quantities of a single item "one at a time", without having to manually set the quantity to one and hitting the E key?

Trading with merchants will increase your Speech skill. The amount of skill progress gained is based on the base value of the item being bought/sold. However, if you trade multiple units of an item in one transaction, you will only gain experience equivalent to trading a single unit; for optimal leveling, sell each item one at a time, and sell expensive items. - UESP.

Not sure about a fix to make it quicker though.

Hmmm... that's a bit of a paradox. I can get a better financial pay-off by selectively looting lighter weight objects with a higher price:weight ratio. But according to this I'll get a better speechcraft skill pay off by doing the opposite.

I feared this was the case.

No.
Put simply Speech levels up the same way as Pickpocket.

The more expensive the item is, the greater the experience. The weight does not matter

The weight doesn't matter at point of sale. But the weight certainly matters on getting the items to the vendor. I make more money per pound of alto wine and gems than I do from most armors. So given the choice I'll ditch the 25 pound iron armor and carry 50 bottles of alto-wine to the vendor. I'll make more money, but I'll have less impact on my speech craft skill.

... unless I sell the 25 bottles one at a time. But I don't know if the sum of the 25 individual transactions would be less, greater, or equal to the speechcraft gain of one sale of iron armor.

hmmmm.....

It would be greater, if the value of the items in aggregate were the same. IE 20 bottles of wine at 20 coins would give more towards Speechcraft than one armor sold at 400 coins. But, the armor gives more than one bottle of wine in a single transaction.

Wish there was a way on PS3 to swap the default option from max amount of items to start at 1 and let you increase how many you want to sell rather than decrease, if that makes sense. Or just get rid of the "sell how many" thing all together so I could just button bash X on my 250 bottles of mead and actually get the skill increases I deserve rather than having to either take ~half hour to sell them individually or just forfeit my increases. Or at least make scrolling from 250 down to 1 faster. It takes soooo looooong...

I find the whole selling and speechcraft malarky highly irritating.

So is with Pickpocket: it's more rewarding to get a heavy object with high value than to steal a light object with low value. But it's even more rewarding to get a light object with high value

Whether you can bring them to the vendor or not is completely unrelated. Speech does not take into account the weight of the object when giving experience

@CCNA:
I'm under the impression that it takes a fixed amount of base value to level up Speech? So 20 items with 20 base value will give the same amount of exp with an armor with 400 base value?

It has pretty much been that way since Morrowind, perhaps before. I havn't played those games.

Your better off collecting the high dollar items and leaving the junk behind.

Collect jewlery, skill books, gems, enchanted items, and crafting ingrediens. Leave the fur armor and iron weapons to rot.

This also has the advantage of allowing you to clear out 3-4 places before you and your mule are encumbered...