Can someone use a trash ability on a card in play, after their opponent has started a trace or do they have to use the trash ability before the trace is started? Encountered this in my first tournament today and was caught off guard by it. Plus side I took second!

Paid abilities of any kind can't be used during a trace, as there is no ability window. The only card I know of that CAN be trashed once a trace has started is Disruptor.

I expect this is probably in relation to a Fall Guy or Cache which has a Paid Ability that can give someone ready cash, which they'd need to beat the trace. In any case the answer's the same: no window for Paid Abilities during a trace.

Cool. That answers that. Second question on top of that. An opponent plays a card that would start a trace. Can I declare before he starts the trace to use a trash or instant ability or that also a no go since he already paid for the card but has not started the trace? I wanna make sure I get this right as it effects how I run my runner and deck design as well.

Learn the timing structure. Memorize the timing structure. Love the timing structure.

https://images-cdn.fantasyflightgames.com/ffg_content/android-netrunner/support/FAQ/Android-Netrunner%20FAQ.pdf

If you are from a MtG background, forget anything about the "stack". It doesn't exist and will bite you when you start assuming it does. There is no such thing as "in-response" in netrunner. The active player can do everything in a timing window before passing priority to the inactive player.

The only thing is a 'prevent' ability which will specifically state when it is triggered.

So for your question, once the corp has used his/her click, there is no paid ability window. The only thing that can prevent that trace is having a lot of money (or Disrupter, but nobody plays that garbage card).

Awesome...thanks. Being a long time MtG player is throwing me off.

I think that's the most common issue for people coming over from magic. No such thing as stack. No such thing as an instant. No such thing concept as "in response".

Whoever has priority in a timing window does and resolves everything they want before passing priority back to the other player.