I keep a book and write down everything that I sell even if it's a packet of carrot seeds.
I always put the variety as well, because it helps to know which are the more popular ones.
Find a wholesaler, as being an allotment Trading Hut you purchase items at wholesale prices then add say, 15% (or whatever you want to) on to sell the items, you'll always have enough there to purchase further supplies.
In my book I have two columns at the end, one for the days trading and one as a running total, so that I know exactly how much I've got in the kitty to spend at any one time.
We purchase from our wholesaler and they don't mind us paying with our own personal cheque and then we take cash in lieu, out of the takings, but this always goes as a "red" entry in the cash book, we have a separate file for the invoices, that way, everyone knows that everything is all above board and correct.
All the plot-holders know that we do it this way, so as long as they're happy, the Treasurer is happy, then it all flows along nicely.
The VAT registration threshold is £67,000.00 so unless your trading hut is going to be that popular (!) then you don't have to worry about VAT.
Our Association has insurance which covers public liability and all sorts of other stuff.
We also have fire extinguishers in the Trading Hut.