im trying to post a add and it keep saying please enter valid demension for your package (applicati.

[ypdc_dennis]

What units of measure are you using?  old style american (pounds/inches) or metric (kg/cm)?

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If posting something for sale on eBay,ca it is usually referred to as "a listing" rather than "an ad".

[femmefan1946]

Also , do the dimensions match the service you have chosen?

https://www.canadapost.ca/cpotools/apps/far/business/findARate?execution=e1s1

A useful page to bookmark.

[momcqueen]

Starting to sell from an account with zero feedback is not a great idea. You will be regarded with suspicion by honest buyers and malicious glee by dishonest ones.

First, do some minor-league buying to gain an idea if how ebay works. It will improve your customer service which is really very important to your long-term success. 

Good luck to you.