Yes, it is possible to sell one of your characters, while you have pending transaction for the one which you bought.
However, this is highly not recommended for the below reasons:
- Even if you set you character for sell the same day you won the char, please note that "The length of the auction must be between one and 29 days from the server save following the set-up of the auction." Source: https://www.tibia.com/news/?subtopic=latestnews&id=5692#ov1 . Now, the person buying the character from you also might take a liberty of paying you only on the last day ( as you mention already, you have 7 days to collect the funds). This mean, that you might miss your dead line and fail with the payment. Additionally, the buyer might not pay at all on the end.
- And the main reason why this is not recommended- The TibiaCoins which you will receive will be partially or completely non-transferable 1- Ergo you will not be able to use them for the purchase of the character for the specific amount of time. 2
1. Tibia Coins you receive from selling a character might be non-transferable up to 120 days after the auction has ended. This feature was implemented to prevent fraud cases. For this reason, it is impossible to exchange them for transferable Tibia Coins before the given time. Source: https://www.tibia.com/support/?subtopic=gethelp&entryid=199
2. Bids are submitted on the Tibia website in the section Char Bazaar. Note that character auctions can only be paid with transferable Tibia Coins. Source: https://www.tibia.com/support/?subtopic=gethelp&entryid=192