There is certainly a market for selling matted and mounted prints, but I don't think bootsales are your best choice of venue. People are looking for bargains and expect low prices, if you want to sell any sort of volume you will need to price them dirt cheap.
You would be better targetting craft fairs, as the people visiting those are looking for handcrafted arts and crafts, and are more prepared to pay decent money for a decent product.
I have also been in retail establishments where one or more local photographers have a display of images, which can be purchased. These range from gift shops, tourist sites etc. to cafes and tearooms (which are good because customers have plenty of time for pictures on the wall to catch their eye). I have also seen pictures for sale in hairdressers waiting areas etc.
These venues will often provide you with a display wall and take a percentage of any sales. Obviously, part of the benefit for them is that it brightens up the room, so they will expect you to produce and maintain an attractive display.
There is no harm in trying a bootsale or two, particularly if it is in an affluent area, but taking a table at a craft fair will probably generate more sales and at better prices. If you combine that with striking a deal with a couple of local businesses, to display and sell your work, you could have a potential business.
How much you can sell will, of course, depend on how good your work is and how well you present it.