how to collect mail from your old house after moving?

You should be able to file a change of address with the post office that will forward ALL mail to your new address. You should contact the seller on eBay and speak with the directly about when the shipment will happen and give them the new address.

Put in a forwarding order at the post office for your mail. Anything sent first class will be forwarded on to you.

You forward your mail through the USPS. If it is being delivered via courier then you need to contact the seller and have them readdress it if it has not already sent.

If it's delivered by USPS then just complete a mail forwarding order. If it's UPS or Fedex contact them with your shipping number and give them the correct address.

Best you can do is ask the local PO to hold your stuff for pickup once you have vacated, but that would be good only for a short-distance move.

You go to the Post Office and fill out a change of address form and they will forward your mail for the next 6 months.

No the new tenants will not keep your mail, nether will the landlord. You have to get your mail forwarded by the post office.