No problem with buying back winners.
If the losers you are selling have other related issues, e.g. selling an XYZ Preferred A and there are XYZ Preferred B, C, etc, you can buy any of the others (B, C, etc) with no problem. You just cannot buy any shares that can be exchanged into the shares that you are selling and have the same rights and privileges, e.g. XYZ.A common shares are sold and there is an XYZ.B common share which is exchangeable for the XYZ.A , or vice-versa, and has the same rights as the XYZ.A as to voting and disolution. Also restricted would be buying call options for the stock sold as that gives you the right to "buy back" the shares at any time even though the option has a definite expiry date past the 30 day period.