For instance what do they dislike or find displeasing when they deal with a customer, or maybe security concerns if any?
Unfortunately, I don't think a Seller will have concerns about a buyer until AFTER the sale is made. A buyer might not pay, so don't send out item until then. Once a buyer gets the item delivered, they could complain the item wasn't in the condition they expected, or even say they never got the item. Use mail tracking for anything of higher value!
What do customers complain about usually?
I have only sold 30-40 items on eBay over the years (normally a buyer) and luckily haven't had any problems/complaints. I assume the most complaints would be slow delivery, no delivery, item not as described, item damaged, etc.
What do customers find exciting about the process of purchasing a coin that maybe was said in feedback or comments?
Unfortunately, eBay does not give the buyer a lot of space (# of letters) to give an enhanced reply about the item in question. There are times when I wrote and paper mailed the Seller an additional reply if I was overly delighted with their item or shipping (I always save the return addresses of any Seller I buy from).
Also do sellers have their own platform here for support or is it topic based kinda thing?
This question leads me to believe you might be asking about a Seller's CCF platform. I have no experience with any sales at this sight; others here might have better feedback on this.