Glad you asked, I had to Google it the first time I saw it.
It is an expression to lay claim to having the first rights - in this case to a potential future sale of the object. On the forum it also doubles as a compliment to a beautiful watch. Knowing it will likely never come up for sale, you express appreciation - and potential interest - by calling dibs, or first dibs, or when it is assumed there are many preceding dibs-es, to be added to the dibs list.
Click to expand...