Is there a way to steal back items sold to a vendor, such as the "Chests below the map" exploit? Could i sell the item then take it back using the exploit? Or once sold is it only possible to get back through buying it again?

Some vendor chests are accessible to console users, so if you sell an item to one of those merchants you could steal it back. Depending on which merchant, it might or might not even officially be stealing. However, if you're willing to use the merchant chest exploit then you can just take the merchant's money, so why bother selling to them in the first place?

because that way i can sell "The right eye of the Falmer" to him (which is not a quest item and perfectly sellable) which is worth 2500 g and then proceed to take it back and re-sell it

This should also work for any expensive item, You could even loot him and sell back his own items.

If you have the right hand of the Falmer you should be so far out in the game that enchanting iron daggers and selling is way more cost effective.
Merchant still has limited gold so then you got it all you has to sell the items to another trader or wait.

Nevermind, ive discovered a way to get atleast 10 000 g every 5 minutes... i sell all my most valuble stuff to Lucen Valerius (who has 12000 merchant gold due to investing in his store and the "1000 gold to all merchants" perk) i then fall through the floor using the wooden plate glitch, and use Do Not Delete chests to make my way to the merchant chest, where i take my stuff back and the gold he has on him, and then take the chests from under me, fall through the floor and hey presto, i have made 12000-14000 gold each time... rinse and repeat ;D hope that helps peoples!

Uh.... but.... what are you doing with all that gold? My latest girl has 100k in her safe.... but there's NOTHING TO SPEND IT ON....