I am exhibiting at a local artfair tomorrow, and have been exhibiting at fairs, big and small, for antiques and art/photography for years. Bottom line is that you need to offer prints in all formats, to sell. When faced with options to buy unmounted, mounted or mounted and framed most people will buy the unmounted print because:
It is the cheapest way of buying the artwork they like and it is the lightest to lug from the fair to their car - so an easy impulse buy
They have been inspired by what it looks like mounted / framed - but want to put the mount and frame on it that fits their decor or marks it as unique to them (even though it may be one of a thousand prints you sell)
You are limited by how many fully mounted prints in frames that you can fit on the stall - unless it is huge and you only have a few images to sell. So, in a perfect world, I would take one framed (with non-reflective glass to "showcase" the picture) of each of my most popular pictures then a bunch of mounted "ready to frame" for each popular image and the rest unmounted - with a flip album they can leaf through to see the full range of what you have in stock without getting the stock grubby and pawed through. This means that the bulk of your stock is "out of the way, under the table" leaving your display space nice and clear for your "samples and examples"
I would also have postcards and greeting cards to sell as people love a souvenir and sometimes want to see if the rest of the family/ board / office like a picture, before buying a full size one. I used to give these as free samples - but can't say that ever worked... so now sell them at slightly over cost...
In my imperfect world, however, I am unexpectedly manning two stalls (one for my art and photos and one for an artist) tomorrow and am woefully under-prepared - so much so, I am here, rather than frantically prepping... However, I have a heap of unmounted prints, quite a few postcards and some original artwork / photo prints left over from other fairs... so not a total disaster... but I wish I had done everything that I am suggesting, right now... good luck!