Hello --

I am trying to write a program that will enable me to create a file that logs instances of a certain kind of object appearing for auction on eBay (for simplicity's sake, I'll call it a "pen") over an extended period of time.

I have my program built to the point where I can get it to look at eBay at the present time and log each pen that is currently up for auction -- creating a string array of each pen auction's 10-digit ebay id number, and saving the array of numbers to a text file in the sketch folder.

But what I can't figure out is how I can add to this file (rather than re-writing it and losing old data) over repeated runs of the program.

Put another way:
1) Say today I run the program, and two "pens" are found on eBay, and their IDs are 34 and 37.  So I save a file in the sketch folder called "auctions.txt" that contains those IDs ("34 37")

2) In two days, I run the program again.  Again, two pens are found on eBay, and their IDs are 37 and 52 -- the auction for pen 34 has ended since I last ran the program, so it is no longer on eBay.  The fact that 37 is a duplicate is not important, but I would like to add 52 to "auctions.txt" without losing the original entry of "34 37".  So I would like it to now read "34 37 52" (or "34 37 37 52"... all that matters is that I have all the numbers).

Any suggestions would be appreciated!  Thanks very much.