If the seller does not ship, by default you will be refunded. If you search the forums, realistically...it will take about 10 buisness days (not including Sat/Sun/Holidays) to see the money ONCE the transaction is canceled.
Technically, OfferUp's TOS states that refunds will take "3-5 business days", but that can also equate to 1 full week, since that does not include Sat/Sun. From what users have specificed, it actually takes a bit longer than what is specificed on the terms of service.
There is no phone support, but you can reach out to shipping support within the app, but it will not expedite the refund process.