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If you're trying to sell a home that you're renting out, make sure that you're on good terms with your tenants. They need to be flexible in allowing investors or other real estate agents entry into the home for showings.

You'll also want to allow them some time to tidy up the home before showings. This can be difficult, but it's very important that it gets done. 
I have a client right now who is having a very difficult time selling a listing because his current tenants are being unreasonable. This is why it always pays to have a good relationship with your renters. 
Another tip I have for you is to follow the Landlord and Tenant Acts, because the last thing you want is to have something pop up and surprise you.
If worst comes to worst and the tenant is so uncooperative that you cannot sell the property, I suggest that you just put the sale of the home on the back burner and deal with those issues first. 
Sometimes just sitting down and speaking with your tenants can help to make the best out of a bad situation. Bring over a pizza and allow them to tell you what is bothering them. Most often, communication is the biggest issue, because tenants rarely have the opportunity to speak with their landlords.
If you're having trouble with this issue, please don't hesitate to contact me. I would be more than glad to give you a hand. 
As always, refer to me as your #1 resource for real estate in Calgary!